I would call it a form of daydreaming. She is aware that her mind can now do more then one thing at a time. It is normal, but since it is new to her she doesn't understand it, or how to describe it. I am willing to bet that a lot of these "voices" happen during school. Some people doodle when bored, others daydream, others get into trouble, etc.
If it is mainly happening at school, suggest to her to start drawing pictures of what the teacher is talking about ( a form of doodling )
Great answer by our Sandman! (as usual). I think we hear our own voice in our head all the time . . . and it's perfectly normal. And it is confusing for young kids. good luck
I have had these EXACT conversation type voices in my head ever since I was a small child. They're never scary or tell me to do something it's more like I'm overhearing someone talk to another person. Lots of times it's only fragments of a sentence, other times its whole sentences. They never pertain to me. I've always liked this unique thing (though I didn't know it was unique until I was an adult lol). Whether you're open to this or not I fully believe we are picking up spiritual messages. I'm very "spiritually" gifted and always have been. Look up clairaudience and see if that makes sense to her. It may not yet but could in the future. Please don't try to rid her of this. Just explain she's safe and if she ever feels unsafe for any reason to let you know. It's kind of like seeing a spirit. Some of us can, most people can't. We're picking up on a frequency that most people are basically deaf to, but we're able to see or hear clearly. It's normal for us. If there's anything else I can help with please let me know. It took me lots of thinking and researching to figure out what this was. It's not your normal "I hear voices that are talking to me" type of thing. It's more like you're a radio picking up sound waves. Hope this helps! She's probably quite gifted as well.