The basic idea with the music system for the battles is that when a battle is initiated on the campaign map, depending where you initiate the battle in the world, the music that will play during the battle will be based on the originating culture for that area i.e. if you start a battle in London you will hear Western battle music. Each actual battle consists of two pieces of music, the deployment (or tension) music and the battle (or attack) music. As you start to deploy your army the tension music will start, it will continue until the game detects that the battle state has been initiated. Once this state is reached the music will slowly crossfade from the tension music in to the battle music which will loop for the duration of the battle.
The final area of music usage in the game is with the in-game movies. Here again as with the rest of the game we have cultural variants of music to match the visual variants of the videos, but in addition Empire features the Road To Independence quest which also required a variety of cinematic sequences and musical accompaniment, this time building an American flavour in the music.