it's a good question when you think about it, because when you think it doesn't really have a sound in my head, the words go in but they dont' really have a voice, now I am thinking trying to figure out the sound of my own internal voice lol
not sure about deaf people that have no language at all, or maybe someone that is raised in the wild by wolves or something, that would be interesting to think about...... do dogs think in yelps, whines and barks?