When does she hear this "voice"? Does it only happen in one place or at one time?
I would give your pediatrician a call. If the voice is scaring her or bothering her, then it is wise to follow up on it. We all have an 'inner' voice in our head but can usually determine it is our own. She sounds like this voice is intrusive and she is not in control of it. I'd try to support her without going over the top to reinforce it and seek help via her doctor for the next step to evaluate the situation. Best of luck to you and let me know what the doctor says.
No advice but I'd call your family doctor and maybe a child psychologist. I hope you find an answer and some help for your girl.