From Halopedia, the Halo wiki
Suite Autumn is the twenty-third track included in the Halo: Original Soundtrack as a suite/medley of four short music pieces.
The tone is somber, and somewhat ominous. The tone adds to an atmosphere of uncertainty. The soundtrack is composed of four sections. The first section (0:00-0:48) begins with low notes which only lasts a few seconds before being cut out abruptly by high strings, creating a suspenseful feeling. The second section (0:49-1:27) enters with two-part low strings playing in ambiance. The third section (1:28-3:01) cuts in with a two-part high vocals, followed by high notes in the background, echoing the rendition of the Halo Theme Gregorian chant from the earlier track Enough Dead Heroes. The fourth section (3:02-4:22) features low strings and vocals in a somber ambiance piece.
The piece was fully reorchestrated into Suite Fall in the Halo: Combat Evolved Anniversary Original Soundtrack.
The soundtrack can be heard: