Interesting question, I never thought that the voice in my head, reading out my thoughts actually had a tone of voice or a persona.
But if I were able to choose who my inner voice would sound like, I'd either want it to be Zach Braff, Donald Faison or John Cleese.
Also, this may be off topic, but it's an interesting thought that our thoughts are like some kind of narration, and although it's our mind, we still have to iterate what we're thinking. I mean, did cavemen think to themselves in grunts and puffs? Or did they just think with images and feelings?