inner ear damage
There is a theory on dyslexia and ADD that these voices are caused by inner ear damage, from infections in childhood. The damage creates disturbances in balance and some sound processing, and that affects auditory thinking. A related theory is that the brain gets around this damage by converting some thoughts into sounds and "rehearing" them with the ears. The process would be similar to hearing a song and then having it in your head replaying afterward except the brain makes the thoughts into a voice then hears that voice. This can also be related to schizophrenia, multiple personalities, hearing voices, etc where someone hears their thoughts like this and thinks they are voices from someone else.