Halo 2 ClassicGame mode:
This achievement is a little tricky as the criteria for unlocking it can be a little sketchy. I recommend grabbing the sword as that's how I unlocked it, but it should
work with any weapon. Now, you'll want to go to the lowest level of the map, and stand as close as possible to the metal door next to the 3 story lift. Sit here for a couple minutes. It could take 2-6 minutes for the song to appear and finish, and it is almost impossible to hear in-game because of the ambient noises in the level.
The achievement should unlock in-game but some have reported only unlocking it once they "End Game" from the pause menu.