I have a couple different voices. If I'm reading a text book, I'll make the voice sound like Cartman, so I'll pay attention. It usually sounds like my regular voice, but when I'm reading a story, Kurt Cobain's voice will be logged in there some where if there is a male character.
I have a slight lisp in my normal speech, but with my inner voice, I don't. I also do character voices in my head when silent reading (it's always made the stories a lot more interesting). And if I quote something said by like Homer Simspon in my head, it's in his voice, and not mine.