Leaks:
http://www.interlopers.net/tutorials/2248Sound problems:
Check that it isn't the game itself. Make a very large, sealed cube. Place a nice, big wall across the middle. Make your friend shoot shit over there. If it's still doing it you've somehow fucked up the game files, possibly in a very simple way. Then check with an official map. If it's NOT doing it there, you've somehow got hammer to fuck up your map and in turn really, really fuck up your sound grid.
If it's happening in an official map reinstall the game.
Basically, if your sounds do odd stuff like that it points to the usual 'layout sound grid' as I call it being either disabled or broken. This is nigh-on impossible to do accidentally. Sounds and what they sound like, i.e. pitch, echoes, etc are judged but the distance to the player and the intervening stuff. Basically, unless you've shanked the map, there's something pretty dodgy going on.