'Tis a splendid idea. A bit hard to pull off, but I would sell my soul to Fraxis if they could achieve something like that.
Another idea, which i think would be a bit simpler, would be to have civ-specific, or cultural-specific music. For example, you would hear hamer-dulicmer songs with polyrythmic beats if you played as India, or China. If you played Germany or England, you'd hear classical chamber-music. Maybe sitars for the Persians.....
What would also be neat, would to have two different tracks- one agressive (for war-time) and one passive (for peace). I think there was something like this on Call To Power.
Doing this would bring the player more of a feel for their civ, and would be cheaper than paying lots of royalties.