That tends to get a bit tricky in PreCU if you want to do it right, due to SOE being idiotic and using generic names that have nothing to do with the weapons, inside the datatables/weapon/weapon.iff, which one has to cross reference to each weapon object .if to see which ID they use.
Fortunially, I already did that in the past for a mod, so here is the list to figure out what is what in the weapon.iff:
03 Lightning Rifle
05 Elite Carbine, Laser Carbine
06 CDEF Pistol, CDEF Pistol Corsec, CDEF Pistol Noob, D18, FWG5, FWG5 Quest, Power5, Scout Blaster, Scout Blaster Corsec, SRS Combat, Striker Noob
07 Beam, CDEF Rifle, DLT20, DLT20a, Laser Rifle, Laser Rifle Noob, T21
08 CDEF Carbine, CDEF Carbine Corsec, DH17, DH17 Black, DH17 Snub, DH17 Pistol, DL44, DL44 Metal,
09 E11 Rifle, E11 Carbine, E11 Carbine Quest, EE3
10 DXR6 Carb, DX2
17 Pistol Launcher
18 Scatter Pistol
19 Republican Blaster, Republican Blaster Quest
20 Heavy Lightning Beam
21 Flame Thrower
24 Geo Blaster
28 E11 Carbine Victor Quest
34 DXR6 Rifle
39 Acid Beam
projectile_pistol - Striker, Tangle
projectile_rifle - Tusken Rifle, Tusken Rifle Victor, Nym
00 Heavy Rocket Launcher
01 Heavy Particle Beam
02 Heavy Acid Launcher
03 Lighting Rifle
Okay, so, as I've mentioned, go to datatables/weapon/weapon.iff, first look for the gun's sound you want to replace, then find the gun with the sounds you want, then look at the Fire Client Effect column, get the .cef name, then in treExplorer look for that .cef, open it, look into the PSND chunk, see what .snd file is attached (.SND links to .wav and .mp3 files). Do the same for both guns, so you know the .cef and .snd name for both.
.cef would be:
There are multiple ways of doing it, depending on if you want to overwrite a whole weapon group or just a single weapon (Some weapons do have unique sounds though, like the Republic Blaster, in this case).
Simplest way of doing it, which would work for the Republic blaster, since the effect is not shared, is to replace the .snd with the one you want.
So for example, with the DE-10 sound to overwrite the Republic blaster, the .snd would be:
If you want to edit a particular weapon that has a shared effect, it gets a bit more hassling, but I can explain it to you in detail if you want to know (Will most likely write a guide for it in the next few days anyway, been planned).