Oh, they can definitely understand him about as well as anyone else whose language he doesn't know (which is only a little worse than those who are speaking a language he does know).

What I meant was the effect of his existence: in most pre-modern cultures (and some modern ones :/) Deaf people have no structured sign language and aren't taught lip reading (it doesn't come naturally) So they have no way to communicate with anyone except maybe their family (and even then only with a limited set of signs) and if born deaf will be seen as mentally disabled.

So you have this group of people who were voiceless suddenly getting a way of communicating with those around them, but only when Vorn is in their vicinity.