From Halopedia, the Halo wiki
The music starts with a light melody of wind instruments and high strings, later joined by the choir. The melody repeats, with the choir The melody then comes to a dramatic climax. Low strings take over, playing a somber melody. High strings and vocals return to join in together for a hopeful melody. Unaccompanied, the high strings plays the five-note E Dorian melody in legato - the melody was featured prominent as part of Finish the Fight, which was played during the Halo 3 announcement trailer. Near the end, woodwind once again takes the melody, which, after a climax of dissonant strings, is replaced by the distinctive five-note E Dorian melody, this time played in piano chords.
The opening wind instruments and high strings were later featured in the track Legend.