Well, there's actually a lot of WDW music that has been released...just not all in the same album. Right now, there is:
4 Parks, 1 World
Illuminations: Reflections of Earth/Tapestry of Dreams
Magic in the Streets (I think this is still out, even though only one of the parades featured still exists)
Music from Disney's Animal Kingdom
Finding Nemo: The Musical
Festival of the Lion King
Pirates of the Carribbean
Haunted Mansion
Magic Kingdom Event Party Music
Holiday Wishes (Includes Jingle Jungle Parade)
Candlelight Processional
Wishes
Fantasmic
Spectromagic
I probably missed one or two, and this doesn't include the music put out by groups that perform at WDW, like Voices of Liberty, Off Kilter and Mulch, Sweat and Shears. For just a general selection of WDW music, you can't go wrong with 4 Parks, 1 World. Then, feel free to mix and match based on which shows and parades you like.