In my mind, of theoretical magicness, I feel verbal incantations are just a way to focus or emphasize the spell you want to do. Like say if you were doing something in real life, that you were still getting to grips with, then you'd talk your way through it as a way to remember what you're supposed to be doing.
That's why, I feel, that you can silently cast a spell that's a few levels below your current ability; because you're that adept at it, you don't need an aid to help you focus on casting the spell. So if that's true, if a child was a mute then they might be able to find a work around to allow them to learn spells.