This is a single 0 byte file with a .ogg extension contained in the appropriate folder structure that overrides (and mutes) the sound made by Inner Focus.
To install it, just extract the downloaded zip and copy/paste the Data 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 it will try to install this as an AddOn (which won't work).
Credit to etgohome of Wowhead for posting this comment with the path to this sound file (even though they suggested using a .wav file instead of a .ogg).
This will also mute the Mage portal summoning sound due to both spells using the same sound file. Unfortunately there's no easy way around this.