wait it out, if she seems ok otherwise she most likely is.
Let it alone , next time she says it be non commital, dont feed into it , it may go away..
I believe her and so should you.
how is her imagination? Not trying to diagnose here but I think I know a way to see if she is pulling your leg..... I have a church friend- her daughter said she talked to dead people. aunts, a cousin that died,etc. They gave her lots of attention because it was so bizarre- they wanted to hear all about it- They started limiting her tv shows and quit giving her any attention about it whatsoever- just - oh? and then changed the subject- She stopped it all in a week.
But if they don't disappear, the auditory hallucinations should of course be discussed with her pediatrician. I won't get into the whole poletergeist business for pre adolescent girls, it's just TOO weird and no one would ever take me seriously again, if they even do NOW
keep us posted- very interesting.