From Halopedia, the Halo Wiki
The Maw is a piece included in the Halo: Original Soundtrack.
It starts playing at the beginning of the level The Maw, the namesake for the song, when the Master Chief and Cortana, riding in a Banshee head back to the Pillar of Autumn.
It is also used as part of Dust and Echoes.
This piece forms the base for the Halo 2: Original Soundtrack piece, Remembrance, and the intro of Ghosts of Reach.