Can somebody explain to me how to customize the music for the game?

[Dark Myrmidon]

The only era I want to customise would be the Modern one, as I think it would be a nice touch to have your favorite modern artist playing in the background. I basically copied and pasted the old music folder, took out the current "Modern Era" music and replaced it, then went into the game and switched the game to the custom music folder. It still plays the old music when I put a custom game on Future... What have I done wrong?