What i done was,
I went into the download (Into the file) and where it says "Player_weapons.rpf" go in there and then open OPENIV and go to your mods folder x64 audio sfx RESIDENT.rpf and then edit mode, Right click then press "Import opemformats" and then open "ptl_combat.oac"
Works perfect for me
And the file name for the actual Firearm sound is "PTL_COMBAT_SHOT_VER3.R"