W.S.
I don't have specific experience with "voices," but I do have a daughter the same age who is definitely imaginative (she tells me all about her "school day," in believable detail--which she does not go to school), so I wouldn't be at all surprised if she is just "pretending." You know it's not that uncommon for children to have imaginary playmates that they treat as though they were real, wanting a plate set out for them at dinner, etc.--especially if she's an only child. I would be extremely careful about what she watches on tv and see if it diminishes. My 4 yo is very very sensitive even to "G" rated movies that have scary parts, and she definitely picks up on things said in the movies, tv (and other kids).
I wouldn't make too much of a deal out of it because that would reinforce it, but you could calmly ask her about who the voices are or what kinds of things they say. You could treat them like you would another child saying those things--like "well, he/she doesn't sound very nice to me, why don't you stop playing with her, you have nicer friends" etc. Let her know she has some control over them, in a way.
If this doesn't help, then maybe I'd ask a professional opinion.
Let us know how it goes!
Blessings,
Lynn