Hehe,
first we need to find FN_FAL pid :
#define PID_FN_FAL ( 351 )
next we need to find weapon sound id of our item(351) :
Weapon_SoundId_0=72
Weapon_SoundId_1=72
(number indicate primary, secondary or third attack of weapon)
Now what to do with this number? It works like old fallout works :
Sound file are named with 8 letter:
1st = W (weapon)
2 = A (attack) R (reload) ..
3 = id, 72 in ascii = H
4 = attack number
last 4 = to have different file, usually XXX1, XXX2, ...
So our fn_fal is linked to WAH1XXX1 WAH1XXX2 WAH2XXX1 and WAH2XXX2. (2 for single shot and 2 for burst)
All is done by engine based on our "72" soundId
Now engine support some audio format (acm, wav, ogg, ..), I don't know if we can use all of them in weapon sound, I think we can but we need to try.
I also don't know if engine use the SNDLIST.lst where all sound name are defined (master.dat)
So the easier way is to replace acm files by new ones. Good luck.
Info on fallout sound :
http://falloutmods.wikia.com/wiki/Making_weapon_sounds