Glad to see that its not only me having this problem.
I use Waffle's sound pack, but all of my gun sounds I created myself. I thought it was pretty good - except no matter what I do the volume levels fail to adjust accordingly - whether by the in-game slider, or by directly editing sound.cfg
So, I had to adjust the volume levels of the wav files themselves. I distributed these gun sounds to my squaddies - and some of them think they are too loud. They also say that they can't adjust the sound levels for the guns.