From Halopedia, the Halo wiki
Perilous Journey is the fifth track in Halo: Original Soundtrack.
The track is composed of an upbeat tribal percussion, accompanied by short rhythmic high strings and long flowing low strings in the background. Near the middle of the track, the low strings plays the first phrase of the Gregorian chant from the Halo Theme.
The soundtrack can be heard:
The track was arranged into the following: