I know what you're saying -- this might be against the Terms of Service. But actually, it's not -- the ToS only mentions "modifying" game files, and since you're simply extracting them from their archives, you're not actually modifying them. Plus, Blizzard actually authorizes the extraction of this music for use in noncommercial machinima, so as long as you use this music and any other assets for personal, noncommercial use, Blizzard has no problem with it. Of course, you'll be stuck listening to the music in bits and pieces designed for looping rather than a snazzy CD set. But if all you want to do is taking a listen to some of the tunes out of game, there you go.