"Hearing voices" could be her way of coping with significant trauma. While they don't seem to upset her, they do show signs that her friend's death has caused her some turmoil. I would get her psychological help immediately. If you cannot afford a private psychologist, some cities have free/inexpensive clinics that offer trauma counseling. Some universities near you may even offer free services as part of their training for graduate students, which may even give her access to newer trauma therapy techniques. If your child has issues with discussing how she feels about her friend's death, maybe look into play therapy! It has been effective in some children at opening up narratives between therapist and child.
Have her see a psychologist (not a psychiatrist who is likely to immediately prescribe drugs) to evaluate her mental health and help her talk out any unresolved feelings. I'm hoping it's nothing, though should it be something exhaust all natural options before resorting to pharmaceutical drugs which have tons of side effects. Dr. Kelly Brogan, MD psychiatrist treats psychological disorders using nutrition and has a program on-line that people can learn from (you can listen to her episode on the Joe Rogan Show). Good luck to you! Kids are resilient and you're a good parent for reaching out about this!
I've learned a bit about the "hearing voices" phenomenon. It's more common than i thought. There are organizations for people who hear voices, primarily in the UK and Australia. People who hearvoices join together and learn from each other techniques that help deal with the phenomenon. They've discovered numerous techniques that are effective. I'd look into that and see what they have to offer. Voices are not always negative. If they become annoyimg or get in the way, then there are ways of dealing with it that people have found. They share with each other what they've tried and what works and what does not. People can live ordinary lives with the phenomenon.
If she's hearing real voices, yeah, that's a problem. If she's talking to herself and that's how she's describing it, we all talk to ourselves in one form or another. I'd say it's worth it to have her see a pediatric psychologist and get a professional's view of what's up. All the best.
Ah, sorry about her friend. That's hard on an adult let alone a child who hasn't yet learned how to be introspective and work through emotions. (some people never do, unfortunately). I, personally, wouldn't be too concerned about this in general. It's not that uncommon to talk to ourselves and hear it as a 'voice' in our head. I don't think so any way. But that she's been through something tough, what about a talk therapist for her to sort through the emotions and to get an opinion on what might be going on. Is she just coping as she can and her 'subconscious' is something else happening with her mental health. I sincerely doubt that there is anything other than a young girl trying to emotionally work through emotions and preteens. You can call your insurance for people that they will cover visits to close to you. Or pay out of pocket for someone you hear of that isn't covered (some of the best child therapists don't take insurance, grrrr). Or ask your doctor for some names. Or your school counselor. good luck