:-\ I may be wrong, but i don't think that's possible either :-\
at least as an addon.
You can however, make a mission using some triggers and scipts, and than some custom sounds, it wouldn't be easy neither pratical, but I don't see any other way of doing it :-\
You can choose the volume of the firing sound, declaring it in the cpp file, you can with a sound mixer make a lot of effects, like distortion or even delays, but I don't think you can make the weapon fire a sound, and at the third person listen to a differnet sound, rather than the effect the game is produced to make :-\
Maybe you could change the effect the game produces, which is something like a fade, but would be really complicated, and not advisable at all