My friend prefers shooting zombies over melee. We play about 20 hours a week together so I hear a lot of shots.
Hee can be 5 feet away, or 300 feet away -- it makes no difference, I only hear about 1/3 of his shots. No gunshot, no bullet impact. Nothing.
I don't know why. I haven't spotted any pattern to it. He's used all kinds of guns, none are any better or worse at properly making sound.
It is terrifying that someone could be shooting at you from 5 feet away, yet you wouldn't hear anything at all.
EDIT: If I had to guess, I would say the pattern is that it's my friend -- or rather that it's someone I've been near for a while. Something about playing with the same person for a while, being near each other, makes it so you can't hear their gunshots. A theory, at least. I don't recall ever running up to a unknown player who shot at me and I didn't hear their shots.