I agree with J.C. there Brez, and as far as "voices?" I think we all have them, it's in our thoughts, I have a perception of how my voice is and it is in my thoughts when I think things out, and I'm sure other people think the same, is probably why most of us are shocked when we hear an actual recording of our real voice on an answering machine for example. When you think of memories with other people, does your mind remember their voices? Of course it does! I remember voices from SFC friends and foes alike very well from the teamspeak and campaign days. Doesn't mean I'm looney.
Now when that inner "voice" is out of tune with your thoughts and starts taking an identity of it's own, THEN I would think someone has a mental illness.
The psych "experts" have an internet dependency illness listed in the books now, most of us would fall into that category, I guess we should all be following life blindly and veg on tele? Of course NOT! Our minds are far more active than most people, and those psych "experts" can take their medical journals and shove it up their arse!