The maps in Quake 3 are a range of space and demonic places and thus, the music respects this and has different types to suit each type of map. This is in contrast to Quake 1 and Quake 2, which have a single type of music.
Quake 3's music is a collaboration of Sonic Mayhem and Front Line Assembly. Sonic Mayhem has released an album containing their portion of the soundtrack, giving previously untitled tracks names, as well as bonus tracks not found in the game.
The audio is integrated into the game's data files, so there is no need to have the CD inserted in order to hear the music.