Another gamebreaking sound bug is a little harder to explain. Most people don't really notice it but I'm sure it impacts their playing as well.

When you're running around and suddenly a zombie screams or somebody reloads the sound does not stay where the source is. when you move your head around the whole sound clip is played on that place in your stereo panorama where the initial sound started. example: you run down the street and on your right a zombie moans for 5 seconds, the sound clip sticks to the right side of your headphones. you make a turn, technically the zombie now moans on your left side, still you can hear that sound clip on your right side.

happened to me with zombie sounds, gun shots, inventory sounds (like reloading, splitting, eating etc.). it started a couple patches ago and really makes it impossible to pinpoint where a player or a zombie is.