well you don't really make a sound when you're speaking inside your mind. I'm not exactly sure what you mean by voices. If the phrases that you hear inside your mind arn't originating from your own thoughts then they are hallucinations (commonly called voices).
It could be you are having ocd or obsessive thoughts. As I understand it, they can be experienced by the person as not originating from the person. If you are not hearing actual sound, maybe it could be that.
i have this too...i get a thought and its not in my voice like i hear my name being called but its in my mind and not in my voice....i feel like im causing it though because its only when i pay attention to it that it happens...i feel like i am just imagining it