The Lost Song

Lost Song is a soundtrack in Halo: Combat Evolved that doesn't appear on the soundtrack. The title Lost Song is a reference that it 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 cross the two bridges with the two Banshees and Hunters. Once you kill all the enemies in the first room the music will start playing. It is played on an infinite loop until you clear the rest of this section, so you can listen to it for quite some time.

It is remade into Arborea Above in Halo: Combat Evolved Anniversary, which is included in the Halo: Combat Evolved Anniversary: Original Soundtrack.