When dying in multiplayer, 9 ou of 10 times when being revived the sound is muffled down and can stay like this for 2-3 minute long if not more. It is not a volume down effect it's really like you where listening to the sound form underwater. There is a kind of audio filter on the audio stream. It does affect every single audio (music, sfx, etc). The sound do comeback every single time just after an enemy is killed. I assumed it's due to piece of code stopping a certain set of audio stream running that happen to clear that filter.