What do your parents say about this with your sister? They should probably be taking her to a doctor to have a thorough evaluation done. Telling the doctor what is going on is the key advice that I can give to you. If the dr can't help you, then they need to refer your sister to a specialist. Only a specialist can help her out. She may only have to go and see a child psychologist or a child psychiatrist. Either way, she will get the help she needs. Good Luck.
Forgot the link.
http://books.google.com/books?id=LG9ctTVMOlUC&pg=PA57&lpg=PA57&dq=hearing+mumbling+voices&source=bl&ots=jAWSdSQhOA&sig=PqBqkRmr4om7uo5D-v5UDeEL5o8&hl=en&ei=cwNzStTXMYGxmAf0nMHTCg&sa=X&oi=book_result&ct=result&resnum=9#v=onepage&q=hearing%20mumbling%20voices&f=false
Bobbie, if she's hearing mumbled, garbled voices that she can't understand, and music, it's very likely to be an organic brain problem rather than a mental illness.
Mumbling and music are usually caused by lesions, tumors, seizures, brain injuries or viruses in the brain. It's a physical damage kind of thing, that can be very minor.
Voices that actually say terrifying or bizarre things that are understood by the person are more commonly caused by mental illness.
This is the only link I can find that kind of describes what I'm saying - go about 2/3 of the way down.
Best wishes. As scary as this sounds when she says this, this is usually okay.