I think Dohl said something about hand signals language, or something, and I just remembered seeing this game a few years ago. A game made by a doujin group named YGC. It is about a mute girl. The particularity of this game was that her "speech" was animated. Maybe it can help inspire the team or learn something?
http://ygchp.hp.infoseek.co.jp/Game/YGC_GAME.htm