It occurs to me, with a (fairly) cheap Muto Imaginem spell/item, couldn't I pretend to be Quietly and Subtly casting?
I suppose you could if you can cast it silent and still.
It's significantly cheaper than the spells I'd LIKE to cast S and Q. And an item that triggers on my casting a spell would be nice.
EDIT: Actually, if I'm reading it right, I can't make it look like I'm doing something I'm not. Creo Imaginem, if so, which is just as well since my arts are similar there.... at least for the subtle one. I could make it quiet with PeIm easily, but then my mouth would still be moving. Greh.
EDIT2: An Intellego requisite ought to handle it.
MuIm to make you appear not to move much and to mask the movements of your mouth. PeIm to silence the sounds you do make. Both effects are probably not that difficult to pull off, although I'd pile up some arbitrary penalties and note that your Voice range will be impaired.
I would rule otherwise, but can't really back it op with RAW. I don't have my books here now.
But! The rules clearly state, that a Range: Voice spell cannot reach any further than the distances listed for sound level. But the target need not be able to hear it, he might be deaf rot the battlefield might have too much background noise. But if you don't say a peep, the spell won't go very far. Right now, I can't recall if "No Voice" means no effect, or just a measly 1 pace.
This is where I put my own two cents worth in. Apparently any natural hindrance does not hamper the spell. So UI'd say that a MuIm spell (or other like it) to make your booming voice sound like lightly rustling leaves would hamper your spell. It would go no further than the resulting sound would carry it.
Same goes for Gestures. Making bold gestures in the dark should IMHO still work. It's not that anybody needs to see the gestures, the magic is affected by the movements. This is a natural hindrance.
But a Magus who is invisible, or somehow alters the species to make it look like he's not moving, should get the modifiers as for the resulting, observed movements. Thus is an unnatural hindrance.
I see this as a question of balance. I play a quite capable ice-based Hoplite. She's much too powerful if also invisible, and can cast without penalty with no observed sound and movements, just becasue of a few, easy Imaginem spells.
I'd say Imaginem spells that affect the range of your voice would affect spell casting normally, both ways. It is thematically fitting. Keeping in mind that the target still need to be sensed, it's not that big a deal.
If you're silent, Voice-range spells can only affect the caster (though they do still need to cross his Parma, unlike Personal-range ones). If you still want to be able to affect somebody else, I think you should look at MuVi spells.
Rego is your techniqe if you want to shift your image. But requisite always needed.
Trick of Secret Casting
ReIm(Mu) 10
base 2, per, diam, ind, +1 Mu req, +1 additional sense, +1 slow reunion
The magus moves his image and sense of feeling back to the distance of the thickness of a hair.
His voice can be heared only to a span. Two spans if he yells.
He cannot use Voice ranged spells under the effect of this spell but can use Touch ranged ones without noticing. The spell also useful if he/she wants to finger attractive member of the desired sex.
When the spell ends the slow reunion option makes it subtle it prevents flickering of the image restoration. The magus may end the spell anytime.
Remove some additions and you get a level 5 spell or master it.
I believe the ability to end a non-concentration spell is coveres by a Mastery Ability. So this spell should run its natural duration of Diameter, unless you master it, and choose this option.
There are some spells in the corebook with this option. For example shapeshifting spells.
Jachra, thanks for the idea!
Those are the spells that require elements such as "a wolf's skin" to work at all. So you'll have to add a similar constraint if you want the spell to be cancelable at will.
It was worth to put that line. 
They really include both the advantage and disadvantage.
It would be a cheap solution to make such a... focus into the spell. So drop this sentence.
Concentration duration would be also fine, however I didn't found the rule in the book how to cast while maintaining another spell.
Sounds similar to Passivity of the Spell Casting Wizard. That was definetely in 3rd edition, possibly 4th.
I tried to do a 5th edition version of it for my house rules (3rd ed saga updated to 5th, needed a few spells)
the discussion is here
https://forum.atlas-games.com/t/house-rules-any-pitfalls-ive-missed/1038/1
I settled for Base 2, +1 Daimeter. Final level 3. Probably far too low for what it can do, but I couldn't think of any reason to make it much higher. Not that it mattered, lol. Saga currently on hold due to work, and no-one cast it 
It's treated as a distraction, see ArM5 page 82 for the full rules on Concentration. Some spells, such as Maintaining The Demanding Spell (are there even others?), are less of a distraction to cast.
Thanks. The ease factor is quite hard. Maybe it would be the best as a magic item. If the magus really wants to be unnoticed he has to deal the event of the reunion of his body and image.
The Baron:
The difference is in the details. As I wrote some elements may be skipped.
Yeh. The version I used didn't have the polish of yours, but was disturbingly easy to cast.
Actually the spell guidelines state that you can cast such spells without the focus, but lose the benefit of being able to cancel the spell at will without it. Perhaps an analogous option for this spell? But I don't think there are any guidelines similar to those in Im.