Our brain works in mysterious ways. We do things all the time and do not even realize that we are doing it. When people talk to us we can visualize their face even though we have not seen their face yet. Even when we see a familiar person face we can hear what their voice sounds like. We can recognize sounds familiar sounds all the time. Whether a person can hear and not see or vice versa there is always a way to communicate and understand.
People can also recognize language through lip reading. When it is difficult to make out what a person is saying because they are talking to fast or to low lip reading can help make out some words. Since I work at a daycare my coworkers and I practice lip reading often. If we do not want the kids to hear us talking about a certain toy or their parents, we will mouth out words to communicate to each other. This way we do not upset the children.