 Seven Halo soundtracks have been released . The Halo Original Soundtrack contains most of Combat Evolved 's music . Due to the varying nature of gameplay , the music present was designed to use the game 's dynamic audio playback engine . The engine allows for the mood , theme , and duration of music played to change according to gameplay . To afford a more enjoyable listening experience , O 'Donnell rearranged portions of the music of Halo into standalone suites , which follow the narrative course of the game . The soundtrack also contains music not used in the game , including a variation on the Halo theme that was first played at Halo 's debut at Macworld 1999 . 
