If you feel compelled to take Diablo III's ominous melodies with you everywhere, its 23-track album in now available for $12/£11 on iTunes. It'll make every walk to the store feel 10 times more perilous -- or, just set the first track to loop to simulate the log in experience, and it's like you're playing Diablo III wherever you are!
Seriously though, the soundtrack was composed by Blizzard regulars Russell Brower, Derek Duke, Glenn Stafford, Joseph Lawrence, Neal Acree, and Edo Guidotti, as well as guitarist/composer Laurence Juber, and it's one of my personal favorites so far this year (hanging out with Sword & Sworcery LP and Dustforce ). Though they work in-game, however, many of the tracks are more ambient than what I'd generally listen to stand-alone.
In addition to the release of Diablo III's tracks, a collection of ear nostalgia titled "The Music of Diablo 1996 - 2011" is now available on iTunes for the same price. Though I think Diablo III's music is great, you can't beat the unsettling sounds of some of those original tracks, especially "Caves" and "Harem," and, of course, the original Tristram theme.