Open your PC's Start Menu. Find Accessories/Entertainment/Sound Recorder. You can open the correct file for the gun and turn the volume for that file down, the save it.
If newer PCs don't have a Sound Recorder (mine is quite old) or you want to change one of the Speech files, which are in a different format, you can download a free sound editor such as
www.goldwave.com, which will do the job nicely.
For myself, I was aboard a ship in wartime, and the guns are extremely, exceedingly, excruciatingly loud, so for me that's part of the realistic experience.