Luck
From Halopedia, the Halo wiki
Luck is a song included in the Halo 3: Original Soundtrack. The song plays only once, during the beginning of Halo 3 in the beginning cutscene of the level Sierra 117.
The song starts with a light melody of wind instruments with choir interspersed in between. The introduction melody comes to a dramatic climax, then turns to a more peaceful strings section. Near the end, wind once again takes the melody, which is replaced by piano at the end.
The song is most likely in reference to the Master Chief's supposed "luck".
It runs for 3:23.
