My eleven year old daughter has bipolar was dx at 4 and she also has heard voice it got so bad that at 7 a voice told her to cut herself and she did.I am not saying this is what see has, but a Dr can help.You can talk to the dr and tell her what she told you and then shecan talk to her one one. Have you noticed anything else like dramatic or unpredictable mood swings does she seem depressed or manica does she have trouble sleeping or are her voice her only symptoms ? I would have a her start a journals on how she feels and what her thoughts are and what going on day to day what the voice say and so on, also you should do the same note what you see if she depressed happy things you observe also talk to her teacher have her write her observation of your daughter at school when you go to the Dr give her these things it will help the Dr and help the Dr to know what going on and help with the dx. Hope everything goes well also a family history will help to.
I have a daughter who is bi-polar and she tells us the same things when not on meds. Have her evaluated, if you wait it will get worse and your little girl will change into someone you don't even know anymore.