The SoundId is an ASCII keys code sheet number ( basically an id for a sound ).
Try opening your _defines.fos file and search for this line " // Weapon <Type Subtype SoundId SoundIdExt X X X [Random 1 or 2]> ". There should be some explanation to the sound system, as I get it, to add your own sounds you have to create a file named properly according to the defined keys in this section. When i was checking it out, i've managed to create a new sound with name like this WAP1XXX1.ogg = Weapon Use ID(80 in ASCII) something xxx( ? ? ? ) 1(random 1 - 2, for diversity, I guess). Place your named file in data/sound/SFX folder or fallout.dat/data/sound/SFX(client side), this should do the trick. And to be honest i did'nt made it all the way though, and I have a question aswell, is this formatting limited to 128/256 unique sounds for each sound type ? I could'nt find what those " X X X " and "SoundIdExt" are there for