Dogs pay a lot more attention to body language than we realize.
We had an old cattle dog that went deaf. He worked just fine on hand signals. Rather than yell around the cattle we'd do an arm wave and point anyway, and he was working without having to listen anyway. So well that I think he was deaf for quite a while before we even knew it!
The only thing we had to worry about was the fact that he couldn't hear cars and trucks, so we had to make sure he stayed off the road.