If you use the following console debug commands:
stat soundwaves
stat soundmixes
You should get an onscreen read-out of how many soundwaves are playing and the Volume Multiplier on each one as well as any SoundMixes that have been applied and their magnitude of affect.
Watch your music Volume Multiplier and see if it drops when you fire your guns.
Watch the SoundMixes section and see if one pops up when you fire a gun.