From Halopedia, the Halo wiki
Suite Autumn is the twenty-third piece included in the Halo: Original Soundtrack as a suite/medley of short music pieces.
The tone is somber, and somewhat ominous. The tone adds to an atmosphere of uncertainty, with the Pillar of Autumn's sharp turns sometimes you never know what's around the corner. The soundtrack is composed of four parts. The first part (0:00-0:48) begins with low notes which only lasts for a few seconds before being cut out abruptly by high strings, creating a feeling of uncertainty. The second part (0:49-1:27) enters with two-part low strings. The third part (1:28-3:01) cut in with a two-part high vocals, followed by high notes in the background, echoing the mood of Enough Dead Heroes. The fourth part (3:02-4:22) features low strings and vocals, creating a somber atmosphere.
The piece is reorchestrated into Suite Fall in the Halo: Combat Evolved Anniversary Original Soundtrack.
The soundtrack can be heard: