Uh, I see what you mean.
On top of the sound pollution, you might want to also reduce the total amount of NPCs that spawn, because at one point my fps dropped to about 1 and we weren't even in combat.
The sound just becomes absolutely terrible, even if one sound is playing and the game starts lagging, and it seems to lag for no apparent reason sometimes. It's unplayable really.
I guess the solution I would do is, besides the lag, make it so only a set number of gunshot sounds can be playing at the same time. I never had to do that so I'm not exactly sure how I would go about that, but that's the best solution I can think of. You simply don't need a room of 20+ guys to each be playing the same sound on top of each other.