I found nothing in Google about modifying sounds for this game. Increasing footsteps and removing ambient noise would be nice.
I got a program called normalize.exe to reduce the volume of .WAV files to nothing and ran this command in cmd.exe to make normalize.exe recursive..
for /F "usebackq" %i in ('dir /s /b *.wav ') do normalize -g -100db %i
I opened audacity and imported all the mp3 files, made them silent (there's a command somewhere to do it), exported and renamed them with Flexible Renamed back to the original file names.
Then I fixed the CRC32s and put them back into my IWDs.
The resulting sound containing IWD files are 17.7MB, I thought this was a success but I still hear most ambient noise.
Are the crucial sound files stored in the .ff files? At least I removed the flashbang ring and some other sounds, but it just doesn't do it.