Sometimes, when I am very tired or sick with a fever, I hear voices while trying to sleep. Maybe it's not really "hearing" them - I know that the voices are not real and that they don't come from the real world - instead I am merely aware of them talking inside my head.
It's as if my conscious brain has been cleft in two. There's a "me" reacting and listening, and a "he" which speaks to me. Sometimes, the voice can become people I know, but it rarely acts like the people it sounds like. It doesn't tell me to kill my parents or anything of such dramatic nature. In fact, I have a hard time remembering exactly what it says. All I know is that it won't shut up. When I have a fever and am uncomfortable, I hear the voice spouting inane garbage, and I just think to myself "Shut up, shut up! I really need to sleep!"
Some scientists believe that before we were homo sapiens, before we were fully conscious, the two sides of our brain worked independantly. The logical, left side controlled motor functions and survival needs, while the right side worked on problem solving and creative endevours. The right side could control the left side by becoming an all-powerful voice: "You there. Go pick up that stick. Now use it to beat that animal. Good. Now go pet the monolith." This past separation of right and left brain could cause voices in our heads, especially in those with schizophrenia, where the connections in the brain are weakened.