I don't know why, but the sound of the shotgun hitting an enemy is so much louder than everything else, especially when firing it underwater. It's also such a terrible sound effect that makes me feel like I'm getting punched in the ears. Is there a way to remove that particular sound effect or make it less loud compared to the other sounds?

Not at home at the moment so I can't check but if you go into your duke3d folder does it have a sound folder in it?

Nope, no sound folder. Just autoload, DOSBOX, DukePlus and samples (which is empty).

Its going to be packed into one of the files then. I'll have a dig around when I get home if no one else has sorted it out for you before then.

Edit:
I had a dig about the sound files seem to be in the .grp file. So to change a single sound you will need to find a program that will allow you to extract the sound file. I believe that shouldn't be too hard to find and do. You could then change it to what you want with an audio editor program. The problem I think will be to repack the file back into the .grp file so it runs with duke3d without problem. This part I was unable to figure out.

Thanks for taking a look at the sound files. It's strange that I was unable to find a post about this particular issue anywhere (on multiple forums), which makes me think either there's some very simple solution to this problem or I'm the only one who's having the problem.

I noticed that if I turned down the number of voices used to 4, the shotgun sound gets a little less loud, but then all the enemy sounds are canceled out.

If you watch some youtube videos of duke3d with the shotgun is yours much louder then that?

So it was a mod you added?

Yeah. It's called DukePlus, and apparently it added extra sounds to the game.