This addon blocks *all* of Doom Lord Kazzak's yells from appearing in your chat window, and replaces his audible aggro and kill yells with the sweet sweet sounds of silence. You'll never even know he's there until the shadowbolt melts your face.
Mac Users: The following process can apparently damage your WoW install. Please place the folders in the proper locations manually. (More info on if/how this occurs would be appreciated.)
Due to how this particular addon works, the zip file contains complete folder structure. To install, you just need to extract the zip to the top level of your WoW install directory. Feel free to poke around the zip to see that nothing malicious is going on. It's just important that the sound files end up in the proper place.
I'm guessing the audio portion will work with any client, and the text blocking portion only with English clients. If you want to localize this into your own language, the code isn't exactly complicated.
To completely remove this addon, you will need to delete the folder that is placed under Data, in addition to the main addon folder. Otherwise, Kazzak will remain audibly mute, while continuing to yell in your chat log.
Concerning the EULA and sound overrides
Blizzard has never specifically stated a position on sound overrides. In fact, at one point Blizzard *did* remove the ability to override sound, but re-enabled it in a later patch, while leaving other overrides disabled.
Without a Blue post, I can not say anything definitively, but for now the general consensus is that sound overriding is legal, and if Blizzard doesn't like it, they'll disable it as they did model overrides. Further speculation on this subject can be found in the following forum posts: