Personally, I think it's something normal, something that all people have, and not something with like supernatural connections. When I hear the word 'inner-voice' I think about the little voice at the back of your head that you can hear everytime you write something, or when you think about something you somewhat build it in your head. I'm thinking about an orange right now, and all I 'see' in my mind is an orange in a black space. It's probably brain stuff, maybe that's just how we are built.
As I'm writing this, I can hear that inner voice, saying all these things that I'm writing. Thing is, you can't ACTUALLY hear it. It's kinda trippy if you ask me, and too complicated to be developed by the FH community (not saying we aren't smart, because we are, it's just that the subject of inner voices is big) . To put it short though, I do have an inner voice. I thought everyone does. Do you like, not hear anything AT ALL whenever you read something, or think about something?