You might find "the origin of consciousness in te breakdown of the bicameral mind" interesting. It references studies as such. It's from 1975, and either a stroke of genius or complete crackpot. But it is interesting, well argued, and potentially mind blowing. I have no idea if anyone studied voices among normal people since this book was published.
Julian Jaynes' book was required reading in my geeky circle of friends, and the root of many "voices of the gods" jokes. Good for endless drunken campfire discussions.
Basically, he said that before we developed language, the voices of the gods ('imaginary friends', 'voices in our heads') told us what to do when instinct didn't. The development of language pushed out the voices of the gods. I don't remember much more than that.
I heard that he had retracted the whole thing, but can't find any evidence of that retraction.
My understanding is that it's not "before we developed language" - but rather, before we developed modern culture, which silences these voices ("imaginary friends") during upbringing.
However, Jaynes argues that various conditions (schizophrenia, non-schizo "voices") are a non-silences version of that natural tendency, albeit often harmful - the revenge of the forgotten imaginary friends, if you will.
