Many recent studies confirm that it is not that uncommon to hear voices and that the majority of those who do are not schizophrenic.
(..)
Talking to oneself is basic to human beings, for we are a linguistic species; the great Russian psychologist Lev Vygotsky thought that “inner speech” was a prerequisite of all voluntary activity. (..) Perhaps there is some sort of physiological barrier or inhibition that normally prevents most of us from “hearing” such inner voices as external. Perhaps that barrier is somehow breached or undeveloped in those who do hear constant voices. Perhaps, however, one should invert the question — and ask why most of us do not hear voices.

//Oliver Sacks, 2012, Hallucinations