First off the soundtrack to the game Minecraft is very pretty in my opinion and is almost the perfect background music
The Link:
http://m.youtube.com/watch?v=NeUP9uMoW4Q
Also The Flaming Lips did a cool 24 hour piece that has many amazing ambient moments in it along with some experimental stuff and a 20 minute really soft near ambient ballad as the intro. Not sure if its what you're looking for but its amazing piece of work that works great as audio wallpapet , but is interesting enough to actually pay attention to as well
The Link pt.2 Revenge of the link!:
http://m.youtube.com/watch?v=C0rIDNtaub0
Also the Disintegration Loops by William Basinski is an amazing piece of ambient music where the sound actually degrades over time a really amazing experience. Again falls into the audio wallpaper but interesting category.
The Link pt.3 Battle Of The Link:
http://m.youtube.com/watch?v=qYOr8TlnqsY