This is a single 0 byte file with a .ogg extension contained in the appropriate folder structure that overrides (and mutes) the new "raid boss whisper" alert sound added in 5.3.
To install it, just extract the downloaded zip and copy/paste the Sound folder into the main WoW directory. If it asks if you want to merge folders, click Yes. Do not install via the MMOUI Minion or Curse Client as they will try to install this as an AddOn (which won't work).
Thanks to Ro for finding the name of this sound in this thread.