I am NOT the original author, I just got the files installed within my WoW folder and zipped them for redistribution, since I can't find the original mod anywhere anymore.
Originally you also had the option to extract gun sounds like 'pew pew' from it, but, since I hadn't installed those for myself, I couldn't include those. This just includes the silencer sounds.
Installation is simple.
Find the 'Data' folder within your WoW installation, most likely 'C:\Program Files\World of Warcraft\Data' if you left those setting for what they were upon installation of WoW.
Once found, extract all the .wav files, including subfolders, to that folder.
(re)start WoW, and all guns in the game which were previously sooo bloody annoying are silenced from now on.
NOTE 1: the content of this 'zip' does not modify any original content of the game.
NOTE 2: the change of sounds only counts for yourself, so anybody you regularly team up with who gets annoyed by the fact you use a gun, should get this (or any other silencer mod which somehow seem to be vanished) for themselves as well.