What is probably happening is they haven't got any code in the game to limit the number of instances of a sound effect being played.
So if you shoot one barrel, you hear the sound effect once, if you shoot 20, it plays the sound effect 20 times.
Pretty basic error, but one that was all too common.
To get around it without access to the games source code is very tricky. Simply modifying the sound effects on disc won't help.
As far as I can see there are two options.
1) Hack the game
This is not a task for an amateur. Would be a really interesting thing to do as a learning process though. A circa 2000 game should be easy to load in something like win32dasm. Then look for the calls to the sound api and work out where they are playing the sound effects. Find a few bytes of spare memory and drop your own code into this space using machine code and a hex editor.
Can be done, I used to do it all the time for a company called Freeloader, but by no means easy.
2) Change the sound drivers on your machine
Also not easy. I don't know what sound card you are using, but regardless I very much doubt you have access to the source code for it.
I haven't written any win32 sound drivers, so I'm not sure if you can get a generic sound driver that will work with your sound card, but I think it's probably your best bet.
Sound programming is one of the areas that I think has been down rated in games coding. There are hundreds of ways of processing a triangle list into beautiful on screen graphics, but very few ways of turning wav files into nice sounds.
Even Final Fantasy XIII 2, or whatever random code the current version is, suffers from this. This is one of the only games that has ever made me want to jam a pencil into my ears and press REALLY hard. To make matters worse, they didn't even put in a volume control for the god awful music. If I tried to release a XNA game without a music volume control it would fail peer review, but Square can do it.