[@TwelveOf8] Giving everyone a learning disability would cause far larger changes. If they can't learn sign language, how can they learn verbal language? Especially given that they almost never hear it. The premise demands that people are intelligent enough to learn a language in the first place. Imposing a learning disability on everyone also greatly limits the players' options for character creation.

(Besides, if you're talking about contrivances, maybe the first thing you should ask is: how do people speak at all without lungs?)

That being said, it makes me wonder what people learn in schools, and how. Since no one will speak, they have to learn mostly through the written word. But how can they learn to read if no one can verbally tell them what these symbols mean? A system of hieroglyphics might be much more valuable to a society of mutes than a system of letters. But then again, no one can create new books, because nobody can write for an extended period of time.

It also, of course, raises the question of why people would bother teaching their children a spoken language at all.

I love this idea for an RP, but it sure as hell is ambitious, having to create the internal consistency to support it.