In unique_items.ltx:

;-----------Daemonion's Audio Changes for LURK 1.2-----------
[wpn_pm_actor]:wpn_pm
snd_shoot = weapons\pm_actor

;Do this for each weapon. It tells the game to copy [yourweapon].ltx and to change whatever you list
;pm_actor is the new audio file, which is referenced here in wpn_pm_actor.ltx