When there is sound coming from multiple sound sources - such as angry infected - at some point the sound starts clipping, as if the game can't handle too many sound sources. Most obvious with infected, if you can aggro more than 10-15 you will get the effect for sure. See the linked [[ https://www.youtube.com/watch?v=x_I9hV_4tKI | video ]], once I have rounded up enough of the infected their screams start cutting out and stuttering.