I know this may seem a lil far fetched but do you think it's possible your grand daughter can communicate with spirits?? It's not uncommon..Maybe you can research it where you live. You can probably find someone in your area that can relate and maybe give you some answers. It'll rule it out if that's not the case...It could also be mental. Has anything like this ever happen in your family line? Please don't think I'm coming from a rude place. I'm just trying to be as realistic with my answers as I can:) Good luck!
Hearing voices is really common in her age group. Here's a recent article - most kids outgrow it.
They sometimes have imaginary friends and grow out of it ..let her tell you about it isf she wants to but make no issue of it what does her mom think ?..