This one thing...

I don’t know what people expect they should sound like, then. Vocalizations that somehow don’t involve expelling air? You can try it yourself: try to make mouth noises while keeping your throat closed. The range of sounds is limited, with no moans or gargles or belches. You can do a few things, like cluck your tongue or suck your teeth, but these sounds aren’t really recognizably zombie-like. If I were the voice actor and OP was the director, I wouldn’t know how to give them what they want.