Sara:
My only intention is to help you find a balanced point of view over this issue. I understand how you must be feeling and your little girl too. I am never willing to share things like these with anybody because I know how one can be misjudged by others, but I will share with you my own experience with voices, so that you don't feel so bad about the whole situation you are living right now.
A few years ago, I was giving a reflexology massage to my eldest daughter, when I heard a female voice whispering something I couldn't understand. I heard the voice between the massage couch and the wall (there are only a few inches of space there), and I stopped massaging my daughter because almost immediately she sat and asked if I had heard that voice. There was no music, no radio, no t.v. that could have been a possible source of sounds or voices.
We both heard it. And that was not my first time. I heard a voice talking to my mother when I was 12 years old. It was around midnight and I was sleeping in her bedroom as it was storming and I felt terrified by the lightning, when a voice started talking to her. I got paralysed but when the voice stopped I called my mom and she only asked if I had heard it too. Frightening...but it happens. I can assure you that I am not mentally ill. Neither is my daughter or was my mother.
It's true that these things happen and that there are people who don't believe it, but they don't believe it because it has never happened to them. That's all.
By the way, like your daughter, I have Scorpio Sun and Scorpio Ascendant, Mercury in Sagittarius, and my 8th house ruled by Mercury (Gemini on the cusp). Our Moons are ruled by Mercury, only that mine is in 8th house (Gemini) and hers in 10th (Virgo), but she has some lunar influence in 8th too. It calls my attention that she has North Node in 9th ruled by her Sun....and Sun in 12th house. 9th house rules visions. She might well be an ultra-sensitive girl, in my opinion.
BUT, there are a number of possibilities which have to be ruled out here, for your daughter's welfare and your mental peace. Wintersprite1 is giving a sensible advice.
First of all, a neurologic/psychologic/psychiatric examination would be desirable to rule out the possibility of a neurologic/emotional/mental disorder. If she is healthy, then you will have less to worry about and you will be able to carefully look for some spiritual help for her.
I wish you and your daughter the best.
Carole