What are the rules for attacking an invisible target with a spell of Voice range and Individual target?
eg Pilum of Flame someone under a Veil of Invisibility, especially if you are not certain if they are anywhere within range.
See HoH:S p.32f Fighting Invisibly, especially:
Likewise, aimed spells can only hit him if the caster knows his location.
He cannot be targeted by spells unless the caster either has an Arcane Connection to him or is able to locate him. Area-affecting spells, such as Arc of Fiery Ribbons, work normally against invisible characters within the affected area.
But before considering specific issues of spell design best re-read the entire chapter.
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To expand a bit: with hermetic magic you are subject to the Limit of Arcane Connection:
Hermetic magic cannot affect an unsensed target without an Arcane Connection.
In your particular example the Target of the spell is the created Pilum, so you can always cast it... but then the problem is of finding the target. In the end it boils down to: can you unequivocally and accurately enough perceive the target or it's location?
All that HoH:S provides is a set of combat/stealth modifiers for invisible characters under particular circunstances, which amounts to "it's harder to find someone invisible, harder to hit the person, and the person has an easier time hitting you", or general rules such as "you automatically identify a target engaged in melee with you". Both of these help answer the previous question (either by giving a direct answer or by making it easier to adjudicate the EF of the is the Awareness roll), but lacking the book, specifics can be adjudicated according to the situation; it does provide some consistency however.
Personally I'm not found of the rule for finding invisible targets as written on p.32, because it specifically requires you to spend a round to try to discern the target's location... and that's not how awareness of your surroundings works. There is no actual action for "hearing really hard" or "looking around really hard", so spending a round to do so shouldn't be necessary. It should just be an opposed test between the person searching and the person trying to hide, modified by the relevant circunstances.
I think that the rule is pretty much what you could expect in real life 
Next time you are in your bedroom and hear a mosquito, measure the time that you need to found it (when you listen to it, so you know that is flying around you and not hidden under furniture). You can also apply it to when you are searching for Wally.
Probably will be more than 6 seconds (or at least, close to them) in each example 
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That is handled by the EF to find the mosquito being relatively high. When I'm spinning around I'm not doing so because it's required to find the mosquito. I'm doing so because I failed my automatic awareness roll and I am still looking around so that I am ready when I finally spot it, but if it suddenly stopped, just looking around would do nothing at all to help me. Thankfully (or not) the mosquito just won't stand still (so it keeps producing noise and I keep getting chances to find it, automatically, every turn).
Unless it stops and I'm actually forced to search for it. Grab a chair and stand up on it. Go meter by meter, combing te ceiling, slowly (this is also what I'm doing when I'm looking for Wally). But this isn't (usually) what I'm doing in the middle of the combat. In the middle of the combat with the beast I'm relying on it's noise to pinpoint it's position. I want my action free to slap it as soon as I discern the location of the foul creature.
I'd say that you didnt failed you awareness roll, since you detected that there IS a mosquito near to you 
But thats one thing, and another is be able to pinpoint the exact place where the mosquito is flying. You cannot do that in half second unless you are very lucky and you find in a very bright and clear of elements (furniture, trees, colors...), and even then sometimes a quick turn from the insect is enough to loose sight from it.
When you really need to pinpoint something thats its very hard to see / you dont have enough cues to know where its exactly, you need to focus into your senses for a few seconds. You cannot find a mosquito while you are dancing, for example. You will need to stop and focus your eyes.
I know some people from Bolivia and other tropical countries. I cound say with confidence that they have a lot more of practices dealing and finding mosquitoes than me Where i struggle to find a kill a mosquito, they can find and kill like 4 of them at a very fast speed xD Its like magic to me xD
But even them, they need to STOP and stay still for a few seconds while their senses adapt to the light conditions and focus into the search item.
You talk about sound, but thats not how hearing work too. You can tell the direction of something thanks to the sound, but the distance isn't so easy. Try to catch a fly with your eyes closed. Unless you are in a film like Karate Kid, it will not work Yeah, you will be able to tell the direction, and when its close or further away, but you will not able to pinpoint the exact location.
Because thats the thing. Not just telling that there is somebody or into which direction. But be able to pinpoint the invisible object close enough that you are able to do something later and not lose it from sight just a second later (you can see how the grass move under its feet, but you need a seconds to tell if its moving or not, and in which direction).
Thats why i think that taking a round to be able to pinpoint an invisible creature its good.
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Two ways to help locate invisible people:
- throwing a bag of non-magical flour in the air, basic, but effective, if water was sprayed around, even better, the flour will stick. However, it is key that both water and flour are non-magical otherwise Parma or magical resistance will prevent the flour or the water to touch the invisible character;
- turning floor to mud using magic (MuTe) and looking for footprint and ripples. Here, I have to admit, I am a bit unsure on how Parma will interact with stone-turned-to-mud magically. My interpretation would be that the invisible character will make ripples - turning stone to mud, although magically - leave still mundane mud.
But here, we are entering the domain of the dreaded pink dot case, so paradox, various interpretations and house rules abound.
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Nope, I failed it. Well, partially.
There are two senses working here, hearing and sight. And as long as you receive the proper stimulus, you have a chance of automatically registering the thing that produced that stimulus. In the mosquito example, it is flying and buzzing around. Maybe I will register the sound, maybe it will fade into the background. We might say that the EF of the Awareness roll to find it is 3 (for example). However, it is small. Even if I hear it, I might not see it. The EF to see is 12 (again, example). If I roll a total of 7 I hear the mosquito, but can't visually pinpoint it's location.
But notice, there's no "action" to hear the mosquito, and neither to see it, really. Both are reflexive things that just happen. Translated to game terms, they require neither a decision from the person neither a deliberate action.
Of course, if I hear the mosquito and fail to find it, I can search for it. But this isn't me just "glancing" around. It requires, at the bare minimum, deliberate intention and action. When I'm looking for the mosquito I'm trying to track it's sound, looking at where I think it's more likely to be based on my previous experiences hunting flying menaces (I have my fair share of those, I live in a tropical country) and shifting my position to better see. It's not just "looking around really hard". STOPPING only helps to focus and reduce your distraction. You can see it as eliminating a penalty you had because you was distracted by something else. But by itself, stopping is not searching.
Looking for an invisible person should be no different, really. I may hear the sound of his footsteps in the grass, I may see the shadow... or I may not. Depending on which inputs I perceive I might not be able to pinpoint the person's location. But this is, again, automatic. It works the same for an invisible thief trying to sneak past a guard, for example. The guard might see a weird shadow or hear something but not be quite sure that it's not just the wind tricking his mind.
Once you have a bare minimum clue (either because you saw/heard something or because you know for a fact that someone invisible is around because you saw them getting invisible) you may decide to take proactive steps to investigate. Which the rules from HoH:S actually touch upon when saying "spend a round searching". But if your initial clue is a sensory input that keeps being sourced by the source, you continue to have a chance to detect it. If the mosquito keeps flying soon or later I will register it's sound. And if I turn my head in the direction of the sound I'll eventually see it, regardless of our size difference, regardless of being actively searching or not (and notice the point I'm making, just turning my head is not active search). If an invisible thief keeps passing through the same corridor again and again, eventually the guard will realize these are the sound of steps, or see the person-shaped shadow on the floor, or both.
Once more: I'm not saying you can't take a round to search for a creature. I'm saying this isn't the first step of the proccess.
I copy from the rules, because idk if im not understanding something, so its easier to refer to it:
It is not necessary to precisely locate an invisible character to know he is somewhere nearby. Invisibility does not make a character any quieter than usual, so rolls to simply hear the character moving gain no bonus or penalty (use Perception + Awareness vs. Dexterity + Stealth – Encumbrance, with no Stealth bonus).
So, its exactly what meant. You dont need to do anything special to detect passively an invisible creature.
You just need to spend a round to pinpoint it. But even if it wasnt necessary and you could eventually see something just looking at the general direction and doing "things"... if you detected an invisible creature nearby, you will literally do nothing to detect it? It doesnt seem realistic to me ^^!
Yeah, you can ignore the mosquito until eventually literally fly in front of your eyes. But we are talking about an invisible creature that its trying actively to not being detected (i mean, it made itself invisible in first place... if it makes insivible just to start touching music wouldnt we would be talking about very different scenario here ). So, unless you look actively, supposedly the creature will just be outside your line of view (or at least, the shadow).
If it just that what you mean, im totally ok with your interpretation.
But thats also exactly what it says the rules of the book... you dont need to use a round to know that there is somebody invisible (you can do that passively, and that step would be the first of the process, ofc), just to pinpoint its exact location.
Detecting an invisible target is not very realistic, because those don't exist (AFAIK) in reality. 
Remember, we are on the assumption that the invisible target is leaving traces behind. There is a faint shadow, and the grass where it stands is pressed against the ground. The traces are identifiable, and not by "doing things", but by using your vision like a normal person.
In essence, it's the same as someone hidden inside a bush. You might hear something, and if you look at the bush, with or without the person making a sound, you might perceive a creature there. You might not know the exact creature, you might even be unable to distinguish if it's a person or an animal, but you have surely pinpointed it's location. And this is automatic. It's how the brain works. For practical gaming purposes, there's no delay. And it may happen even if you are engaged in another, completely unrelated, activity. Remove the bush and the image of the person and you can still rely on any auditory and visual clues as usual.
And if you think you heard or saw something but you are not sure, you can get closer, change your position, look again... and even probe it with a stick, if you are close enough.
But ok, there's a disconnection in our understanding.
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So, if I understand the above exchange, it is possible to casually detect that there might be something amiss in the local area (a mosquito buzzing, or an invisible person humming), but then you have to spend time and effort to actively search for what provided the (possibly subconscious) cues.
The active search might, with a high enough roll, locate clues to suggest the approximate location of the invisible person (shadows, reflections, a moving imprint in the grass, etc).
But is this approximate location sufficient to target a magical strike? Maybe if it is an indirect attack (ReTe sling spells) in which the odds are similar to a mundane arrow, but a Targeted spell like PoF?
I recall classic the DnD phrase "I target the darkness with my Magic Missile".
Is it the same or different for Ars Magica?
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By RAW, no.
In order to target an invisible character with spells or missile weapons, an opponent must be able to locate her to within a half-pace or so. Usually this is done by visual cues (shadow and footprints), but when these are faint or non-existent, it is possible (but difficult) to locate an invisible character by sound alone. (...)
To locate an invisible character accurately enough for spells or attacks, spend a round searching and make an opposed Perception + Awareness roll against the invisible character’s Dexterity + Stealth – Encumbrance + Stealth bonus (from the Effects of Invisibility table)
The bold parts are marked by me. The full rules are detailed in the book ArM5 House of Hermes: Societates, pg. 32 - 33.
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In my contrariness, I find myself thinking of the edge case where the invisible person is also levitating, so they are 2 paces away from the footprints of their shadow.
Then probably you should use your hearing alone to detect that person
Perception + Awareness roll against the invisible character’s Dexterity + Stealth – Encumbrance + Stealth bonus (from the Effects of Invisibility table). When locating the character by sound alone, use the Stealth bonus from the “No signs” row of the table.
The Stealth bonus would be +12 (unless SG says otherwise, if the invisible person is talking or making noise, for example).
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