From Halopedia, the Halo wiki
The Lost Muse is a track that appears only in Halo: Combat Evolved and not available on the official soundtrack. The title Lost Muse is a reference that it is "lost" from the soundtrack. It is remade into Arborea Above in Halo: Combat Evolved Anniversary, which is included in the Halo: Combat Evolved Anniversary: Original Soundtrack.
It is played during the level Assault on the Control Room right before the bridge that overlooks the Control Room. The player must cross the two bridges with the two Banshees and Hunters. Once the player kills all the enemies in the first room the music will start playing. It is played on an infinite loop until the player clears the rest of this section, so the player can listen to it for quite some time.