The music in Planeshift is almost entirely non-diegetic tonal music. It doesn't have a source. The tavern music could be thought of as coming from ~somewhere~, but really the tavern music is just an attempt at having something appropriate. The cavern music has sound effects in it which could have been produced by things happening in the cavern. Besides the cavern though, soundscaping is left to ambient sources and emitters.
For the most part, the music is orchestral. But there's two big exceptions besides the tavern music: The arena and akkaio. Both of those seem to have more world influences. The arena music has definate similarities to aztec flute choirs, and akkaio's music seems like it's trying to create a more Greek / mediterranean sound. The strings in it though are more of an imitation of how western orchestral music imitates middle eastern music, if that makes sense to you.
Is there an overall philosophy to how the music is writen and chosen for Planeshift? Is it selected based soley on overall effect? Are there specific guidelines that the composers are asked to adhere to, etc? Is all the music in midi simply a pragmatic issue, or would Planeshift have real recordings given the option?