Hi Serena,
A good answer from Christina, I think. I'd just like to add another comment regarding children hearing voices. It is actually not uncommon for young children to say they can hear voices talking in their head. Sometimes this happens at certain times of the day - for example just before the child is going to sleep or very soon after they wake up. Sometimes the child is using language to describe certain thoughts they are having, but their choice of language makes us sit up and take notice! Children can also have very active imaginations which can spill over into their 'real' lives.
Voices then, could be an extension of fantasy life. As Christina says, another explanation could be schizophrenia. I think if the voices are not hurtful or telling the child to do things, they may not be a problem. But yes, get the child checked over if you feel health is being affected.