From the official site:

We've updated our music player with a new track, "Power of the Horde" by Tenth Level Tauren Chieftain. Originally featured in Warcraft III as a hidden song, this blistering track can also be heard during the band's live set at the Darkmoon Faire.

You can also listen to a selection of other music from World of Warcraft, Warcraft III, and Warcraft II. Additional World of Warcraft music is available at the Blizzard Store.

The WoW soundtrack is selling in the Blizzard store for $9.99!