The Halo 3 Original Soundtrack released on the 20th of November in 2007. Martin O’Donnell and Michael Salvatori used this soundtrack to revisit the old themes and record them with a grand 60-piece orchestra. Also a 24 voice chorus was used for the recordings at the Northwest Sinfonia at Studio X in Seattle. Revisiting the old theme’s as well as making some new ones.
Read more here:
http://www.haloarchive.com/sheet-music/halo-sheet-music-scores-of-halo-halo-3/