Instructions
Double-click on the "Computer" icon on the desktop and then open the C: drive icon.
Open the "All Programs (x86)" folder ("All Programs" on 32-bit systems), open "Firaxis Games," open "Sid Meier's Civilization 4," open "CustomAssets," open "Sounds" and then open the "Soundtrack" folder.
Drag and drop your MP3 files to the folder.
Launch the game by clicking the "Start" menu button, clicking "All Programs," clicking "Firaxis Games," clicking "Sid Meier's Civilization 4" and then clicking "Launch Civilization 4." As the game launches, the MP3 files stored in the "CustomAssets" folder will override the default soundtrack.