Well, I believe the author of the OP has enough angles on this subject as it is. Also, it's pretty clear that handling sound can be done in different ways, and the newly created house-rule guidelines were working on here, could be done in different ways.
Some would boldly insist that all Forms could potentially do anything and everything given the right amount creativity and reasonable magnitude adjustments. Thus sound should be able to do in Mythic Europe whatever sound can actually do. So if you want to create a spell that causes vibrations in a bridge at just the right amount of Hz employing Mechanical Resonance to destroy it - then go for it. Others would claim that Imaginem is a fickle art, and thus sound should be treated with delicacy and would flinch at your every example from the modern would. They will tell you to look at the way illusions has subtle effects on the world, and use that as a source of inspiration. And from what i read above, apparently the latter are extremely boring individuals
Anyway, you've now seen at least two different approaches on either side of the argument. And please note that although these forum discussions tend to show rather extreme opinions, the above posts actually don't. Noone has claimed that sound should be the new, all-destructive Form capable of matching the damage of lightning bolts. Nor has anyone told you to let go of your sound spell fancies, since it's not really in the books. As a SG you could use the above for inspiration and find your own stand. As a player, feel free to use the arguments above to convince your SG, and push things into your direction
@Direwolf: I'm not sure you got the part about the snake, I'm not talking about how the snake actually percieves its surroundings (according to modern science). I just do not like players to use modern knowledge for creating spells. If a Mythic Europe snake finds its prey in the dark, it's probably because it can see in the dark, period. Or maybe because it has an extremely fine-tuned sense of smell... Or maybe that ever-working tongue is actually tasting its way through the unseen world, that would be creepy. I would personally say that this kind of reasoning fits into my version of Mythic Europe. Whereas hidden, temperature dependent heat glands positioned on the creature's forehead seems well outside the general thinking of that time; thus I don't like it. If it doesn't feel wrong in your game, why not allow it. Invent two while you're at it. But in the stories we tell, we like to keep things simple, through a 'dark ages' approach. My SG wouldn't let me shapechange into a Monkey or a Leopard (we play in Scotland), although these naturally exist. It's not about what is 'true', nor is it a fear of anachronism - it is a matter of style, flavor and mood; a concious choice that we've made for our saga.
The debate on how to leverage sound in ars has been fun. Apart from needing some guidance for my own games I think there might also be real value in the thread for others here too. When the questions on damage started to get down into mechanical detail I took to the internet to dig through the aspects of sound. Its fair to say that just considering the pure volume (decibels) is not the right approach as pitch/frequency play a very large part in any application for sound beyond just loud noises.
Essentially the ars requirements to gain special effects from sound should be higher magnitude than typical "damage" forms, they should be not as scalable, and should be less reliable. To counter this I think the effects should inherently affect an area around the target of the spell, and that the effects of the spell should dramatically degrade as away from the target. This is not because of a imagined game balance reason (which I disagree with), but instead because the complexity of the acts themselves. The acts of deafening a person, shattering glass, or inflicting damage with waves of sound would even be tricky to do today given all our tech, so I see that as the basis for Imagonem: Sound being something to develop with care. I don't think it would require a specific virtue to do, unless that virtue was to somehow make it easier to produce the effects. A magical focus in sound would be very cool.
Finesse has a fundamental role to play in magical sound, for the targeting near the individual (as normal), and for the control of sound to a specific effect. In fact to get a conjured sound to break glass is trickier than I though, with the sound needing to be at least the volume of a very loud yell, and also at a very specific pitch. So its hermetically possible, but far easier to just use PeTe the glass.
As an update to some guidelines I'd now suggest
Base Imaginem 1, to create a sound, likely to be at a volume appropriate for R: Voice; as per ArM.
Loud Sound - a very intricate image is +1 mag, so a very loud noise would be +1 mag too. Akin to a shouting human, or the roar of an animal.
Shatter Glass - based upon a loud sound (above), I think it would need an additional step +2 mag to break glass. +1 for the complexity, and +1 for the additional volume needed. There is also a Finesse roll needed of say 9+. My quick digging suggested that 100 dbs is needed, which is very loud, and probably on the human limit. Humans can however do this, so it should not be set at a prohibitive level.
Deafen - based upon a loud sound (above), an additional +2 mag of volume could deafen people very near the effect (but that sound would not break glass). This will provide a noise level far exceeding what an animal or person could produce. The sound must also be targeted at the head/ears of the creature, which requires a finesse check, typical to aiming a missile. A stamina check of 12+ is needed to not be deafened.
Damage - I'm not sure on damage anymore, except to say that I think it should be possible. Lasse Rohde made a good point about while sound is natural, it is not a substance which inherently causes damage (in the bit with the Danish play). I suggest (level / 2)-10 as the basis. This allows sound to inflict a +0 sound effect as level 20 base, which is then modified for R, D, T. Also means that it is well limited to not ever reaching the destructive power of fire or lightning.
I think you probably go TOO far here with dropping the damage. To get the same damage as from a regular PoF, you need a Base 50 by this...
I would also say that sound DO inherently cause damage, it´s just not always automatically physically lasting.
The shock damage from sound is heavily exploited through use of flashbang grenades, German WWII classic looking stick hand grenades were specifically made to cause noise first and shrapnel second and used more for stunning than for killing, when muskets came about, those armies that didn´t trade long range fire but instead walked up and fired salvoes right up the nose of the enemy repeatedly won over vastly greater numbers, simply because the noise wall of 3 ranks of musketeers firing all at once at less than 20m causes shock and even injuries on people not actually hit.
And if you´re seriously exceptional with Kiai-jutsu, you can literally hurt people with just the voice, if not so good, you can stun with it.
Why less reliable?
I would say that it´s not less reliable, as long as conditions are identical, with range and protection making a lot of difference for very small changes.
Yeah well if you look at it like this, if you can´t do it with Im, then your magic cant actually replicate reality because Im is what you manipulate sound with.
Although the thing about knocking over a bridge by resonance? That´s about as much reliant on the bridge not being good enough as it is with the sound.
Ah yes, but then you fail to accomodate the fact that this "world" includes magi who can investigate things as well as the best of modern marchinery and sensors. And actually also has the opportunity to do it. So essentially, anything that might interest a magi once in the past, might be known as well, or even better than it is today.
Except that "period" will only last until someone asks the question of "how?" and the proceeds to investigate it.
Mmm? Why ever not... Sure i sometimes loose track of threads but i still disagreed with your basic "gameworld assumptions" so to speak, while also having something i could say about it...
Mythic Europe is not the same Universe. Assuming that Mythic snakes have infrared sensors is the same as assuming faeries exist in the modern world. Your Saga may use the same physic, but in another Saga a magus may find that snakes really see with their tongue.
And if you want to place your game in a world where everybody walks on their hands, that´s just fine as long as you dont ask me to consider it a good idea.
I would just point out that modifiers for "size" apply to sounds too. Thus, without extra magnitudes you cannot create a sound louder than that produced by an adult human (see corebook, p.143). Every magnitude increases this by a factor 10, as usual, so you need 3 extra magnitudes to create the sound produced by 1000 men. If you were to take the "modern science" approach this would mean 1 magnitude = 10 decibel. This means about 3 extra magnitudes to cause pain, 4 extra magnitudes to have a chance to cause permanent damage by brief exposure -- something roughly like Jupiter's resounding blow, or a modern gun fired close to your ears.
But what about Ars Magica "physics"? I do not think sounds should be able to affect anything physical such as, say, glass. They are just sensations.
Deafness of humans and animals is another matter, however.
One could argue that loud sounds do not "physically" damage the organ, they just overwhelm it with sensation in the short term, and/or produce something like assuefaction in the long term. After all, a dish with very, very hot spices will leave your taste dulled for a while, and if you always eat very spicy food, "normal" food eventually tastes very bland. Pliny the Elder, if I'm not mistaken, claims that fishermen working near the Nile waterfalls often lost hearing through exposure; and Cicero certainly uses the same example in the Somnium Scipionis. So I would allow very loud sounds to deafen.
Mark Shirley correctly points out that CrIm spells are not resisted -- and despite the views of other posters on the issue, I agree both that this is the RAW, and that taking the opposite view opens all sorts of problems. However, I do not think that it's so game breaking to allow a Level 15 CrIm spell (Base 1, +2 Voice, +4 "size") to have the same effect as Jupiter's resounding blow (CrAu 10). Note that, if you allow loud sounds to deafen, then Jupiter's resounding blow, by the RAW (in particular, by the rule that species generated by magical creations need not penetrate) need not penetrate: the thunder is magical, but its sound is natural.
