I don't think that hearing voices make you crazy. At least that's what they tell me.
Anyway, a lot of things can mess with your brain and cause temporary insanity so to speak. Stress and lack of sleep are two that can make people, including me, have hallucinations both auditory and visual. However, when people start acting on the voices or believe they are real, then they may be on the verge of insanity. When I have these hallucinations they are more just sounds not so much directions or conversations. It can range from a baby crying to people talking around me.
I agree that people who are schizophrenic often believe they are invaded by aliens, demons, or other entities. This is caused by the paranoia which often accompanies this illness. An interesting thing I learned the other day is that depression and schizophrenia are closely related. So that could even be a cause for hallucinating also.
I find the brain very interesting and have research multiple personalities and schizophrenia, along with depression and anxiety. I hope the OP is expanded to allow for more discussion on the topic.