Ehw, I think there are quite some to think of..

But the most significant one would be imagening like every object engending sounds when they move or collide with another object.
Be it a punch from Street Fighter II, a jump from Super Mario World or something else. I've assumably been doing this like all the time I've lived so far.

Sometimes I actually speak the sounds out loud what makes the people around gaze at me.. what in turn is extremely annoying, but well, guess that's what I get