Most of the albums available were included in past Collector’s Editions of Blizzard games, such as the original soundtracks of World of Warcraft and The Burning Crusade, and even the Warcraft III: Reign of Chaos soundtrack.
Here’s a link to the iTunes Store page. This is pretty neat, I think. I was not looking forward to extracting the music from the game files and filing in the information manually for each track. Also, this little morsel is there:
Looks like I’m late to the party. The other tracks seem to have been there for a while. They’re all iTunes Plus tracks.
via Wow Insider