From Halopedia, the Halo wiki
Suite Autumn is the twenty-third track in Halo: Original Soundtrack.
Although the track is a suite/medley of four short music pieces, there is a consistent use of somber, and somewhat ominous, tone throughout, creating 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 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 arranged into Suite Fall in Halo: Combat Evolved Anniversary Original Soundtrack.
The soundtrack can be heard: