Choose Wisely
From Halopedia, the Halo wiki
Choose Wisely is a song included in the Halo 3: Original Soundtrack.
It starts on a single note, followed by a choir melody. The orchestra continues its accompaniment, then choir gets louder and continues its melody (Note: This choir melody is the same as the beginning of the Halo 3 Announcement Trailer).
It runs for 1:19.