From Halopedia, the Halo wiki
Roll Call is the twenty-fifth track (Disc 2 Track 10) included in Halo 3: Original Soundtrack, and plays during the Halo 3 credits, which is alluded in the title of the track. It is composed of Roll Call (0:00-1:10), Duty Bound (1:10-3:48), and Price Paid (3:48-5:57).
Roll Call (1:10) is a reprise of Familiar Ring from the earlier track Gravemind. It starts with a low note by orchestra, quickly followed by a choir melody. This melody is almost the same as the Gregorian-style chant from the Halo Theme, but it has slightly different chords at the end of each phrase, giving it a more uplifting mood.
Duty Bound (2:38) begins with a reprise of Farthest Outpost, including opening muted drums beats. However, after the percussion takes over with strings briefly playing the melody of Weight of Failure (from Prologue in Halo 2: Original Soundtrack), the melody does not resolve. Instead, it transitions into strings playing the opening string melody from the track Follow Our Brothers (itself being a rearrangement of Under Cover of Night from Halo: Original Soundtrack), joined by the bassline of In Amber Clad (from Halo 2: Original Soundtrack). The piece ends with the bassline and percussion swelling to a climax. A shortened version plays in the menu of Halo 3: Mythic.
Price Paid (2:09) features a more minor melody by piano, with strings accompaniment. The piano melody continues to swell, then fade again. The piano plays a mournful yet hopeful melody, and slows down at the end, with high strings holding the last note.