Wow, yeah, I definitely do this kind of stuff.
Particularly the out-loud phrases, funny voices, nonsense songs, and sometimes... just... sounds?
Usually it happens when I'm walking, thinking very deeply about a problem, or showering-- in other words, when my mind is off somewhere else.
While this habit has certainly resulted in a fair share of awkward moments, I think that it has also really helped me in learning foreign languages: the sounds and words just get stuck in my head and subconsciously processed over and over. It's quite fun, actually.
Lastly, no one else seems to have mentioned it, but I have a fairly distinct memory of the day that this outspoken internal dialogue began, and being confused/put off by this new voice in my head. If it weren't for the fact that all of you share this same habit, I'd think it was nothing other than early-onset madness...
Cheers,
Cameron