It's quite easy really. All you need is this table found in tactical\configs\game\listofcreature.txt. All this table tells you is what creature usess which sound (if any) when they are hit and when they die. So if you wish to add a new wav file for the alien you would have to: put a new wav file here
"share\sound\wavs\mutants\" then you'll have to edit listofcreatures.txt and add the appropriate path and the new wavs name to the alien you wish to change.
If you simply wish to replace a wav file let say Car Crabs death sound wich is growl3.wav you would just rename your wav file to growl3.wav and overwrite the original here "share\sound\wavs\mutants\". Then just repack the two files and you're done. The mutant wavs are using IMA ADPCM 44,100; 4Bit; Mono settings. I haven't tried any other codecs or setting 'couse I found long ago (and on other games) that using the same settings as is originaly always works best or even at all.