Although there is lacking evidence that Michael is mute, so why does Michael Myers not talk much throughout his films? One could deduce that it could be because of his time in the mental hospital after he killed his sister, but the reality is that he is a fictional character. The answer is that movie producers wanted to make him as frightening as possible by using several odd disconcerting human behaviors that would leave audiences terrified.
The Psychology of Scary
Deep in our human psyche we understand how 'normal' humans behave. Most communication is done with our bodies and facial expression. We can't see Michael Myers face because he is wearing an expressionless mask which is creepy all on it's own but then when they make him mute too, then you really don't know what he is thinking and feeling and the uncertainty is what makes us feel so uncomfortable. Watch this baby go from happy to terrified when his mother doesn't speak or show emotion on Youtube.