In addition to the civ-music that Nutty described, there's also unique music for each leader. (1 war theme, and 1 peace theme for each leader to be precise).
These can simply be copied over from another leader:
where the sound-files are .mp3 files that have VFS=true, and the leader's type is LEADER_TRL_KING_BOB_OMB
NOTE: Be sure to check the 'reload audio system' checkbox in your mod's properties.
You will probably have to play around with the volume settings a bit to ensure the music is not too loud/quiet
In case you're wondering why such code never appears in the base files, Firaxis uses a different format for audio files that we cannot use. I.e. referencing to the base files does not work here.