While discovering that some gun sound hurts your ears and intended to lower this loud sound, refer to these simple steps to fix it.
1. Obtain a Free Sound editor - Audacity. Here: http://audacityteam.org/download/windows
2. While you're on it, open a file for sound editing. Example: Mp5, from the Sound file in your game move into its farthest files which mostly contains all of the different sounds involved. For Mp5 then its: mp5_clipin, mp5_clipout, mp5_cloth, mp5_deploy, mp5-1, or mp5-2, so and so.
3. Open mp-1(Shown on pic), then go to Effect, then to Bass and Treble(drop-down).
4. From there simply adjust the sliders to your liking. If it's too loud lower the level, for high pitched increase the bass, same works with the treble if it's too dull. Experiment.
5. Then back up the original file for safety purposes and export your new edited sound file to replace the loud gun fire sound or the one you wish to change.
6. Test it, done if satisfied.