Blizzard has added a page to their online store where you can buy all sorts of music from the Blizzard titles. They’ve got all the WoW music you could want, and Starcraft stuff as well, but as we’ve come to expect of Blizzard Irvine, there’s minimal support for the Diablo franchise.
The only Diablo-bolic offerings thus far are the D2 soundtrack and the one D3 single they released (for free) last year. Obviously there will be more D3 musical options once the game is released. In the meantime you should check out the Diablo 2 music and Diablo 3 music pages in the wiki. They’ve got lots of info on the music and sound for both games, plus there are links to get the various environmental tracks for D3 that you can hear in the game and on Blizzard’s Diablo 3 website.