With the release of Legion, including the 7.0.3 pre-patch, Blizzard has added several new sounds for each class which are heard when you use several abilities. To name a few, Beast Mastery Hunters' Dire Beast and Bestial Wrath, Survival's Spitting Cobra talent, and Warriors' Battle Cry and Ignore Pain abilities.
There may be a few more, but Dire Beast/Bestial Wrath are certainly the most irritating ones, as you will be constantly hearing that horrible shout every few seconds. I personally couldn't stand to play my BM Hunter anymore because of it.
To install, simply extract the Sound folder to the root of your WoW folder.
