The Lost Song
From Halopedia, the Halo wiki
Template:Ratings 200px Lost Song is a song in Halo: Combat Evolved that doesn't appear on the soundtrack. The title Lost Song is a reference that is "lost" from the soundtrack. It is played during the level Assault on the Control Room right before the bridge that overlooks the Control Room. You must go across the two bridges with the two banshees and Hunters. Once you enter the room after and kill all the enemies there the music will start playing. It is played on an infinite loop until you leave this section, so you can listen to it for quite some time. Note that there are Covenant in the next hallway and room, including a pair of Hunters, that may pose a threat in harder difficulties.