I think the music of Civ could be better. Sure, it's got Beethoven, Bach and Mozart in the play list, but the music itself hardly reflect what's going on in the game.
I would've liked a dynamic play list that looks at the player's civ, era, civics, state religion and whether or not you're at war and base the music on those factors.
Perhaps those static mp3 files could be replaced entirely by such a system. No more chanting monks in the Middle Ages, unless you're actually running Christianity. Your background music would evolve along with your empire.
Police State + Vassalage + Theocracy should produce ominous 'war is about to break out' kind of music, which turns into a full orchestral war drum rhythm (or techno beat, depending on era) when you declare war and invade your neighbors.
Pacifism + peace = peaceful music. Pacifism + war = scary music (to warn the player that his civics are not effective)
Environmentalism should add whale sounds and songbird sounds to the mix. Combine with pacifism and you've got yourself some relaxing New Age music to reflect that you're running a civilization full of hippies
And some more civ-specific sounds would be nice: If you're playing as Aztec or Inca your background music should have some pan flute in it. If you're Arabia or Persia you should hear some Middle-Eastern instruments, etc etc.
Replacing one instrument with another should be no problem for a dynamic music generator. And this would also prevent that every civ sounds the same under the equal civics (because every civ/ethnic group would have a unique instrument and there is religion to influence the music).
You could get some very exotic sounds. Arabic prayers with Japanese taiko drums during Tokugawa's jihad, Gregorian monks singing to the tune of South-American pan flutes or Indian sitars combined with pacifist/environmental New Age sounds for Gandhi. That would be, in the immortal words of the great Alexander, 'way cool.'