Songs are audio files. Unlike sounds, songs loop forever. You can play any song from Bitsbox's library, and you can control its volume.
This command works a lot like sound(), except songs repeat forever. At any given time, only one song can be playing, so if you call song again, any old song will stop playing before the new ones starts.
You can pass in a volume number between 0 and 100. If you don't pass in a volume, it defaults to 50%, since in most games you want the music to be a little more quiet than the sound effects.