Some tunes by Yoko Kanno... parts of her Earth Girl Arjuna score, some tracks from Cowboy Bebop (the upbeat ones as well as "Rain" - sung by Steve Conte... he sounds more suffering than Mai Yamane), parts of Wolf's Rain and some of her Ghost in the Shell scores, too.
Yuki Kajiura also has composed some nice tracks for Noir as well as for .hack - Legend of the Twilight...
Oh, and some psytrance, too - including my favourite track by Ekoplex called "Seeds" from his album Creatures of the Forest. That track features a well-known voice sample: "Seeds of the sacred tree... very pure spirits"
phrasing it as not being able to understand human music is a bit incorrect i think. na'vi would be able to recognize what human music is. they can recognize melody and what singing sounds like, even if the instruments and language are unfamiliar. they would understand what and why humans make music, to a point. the actual thing is that for them, music for its own sake and the idea of spending a considerable chunk of your time studying it would be seen as a waste of time.