I know that the game can get very overwhelming sound wise when there's hundreds of sound effects that want to go off every second in a highly populated room, thus after going through a few of these very quickly, individual sound effects stop playing and only 2 or 3 sound effects play every 2 seconds or so. 

It gets very weird when only one out of every 4 or 5 gunshots get played, enemies explode silently, and there is an eerie silence when an entire room of enemies blow up.