Tip: Make sure that ALL exit-enabled pipes, doors, etc. have a screen exit defined, or they'll take you to Level 0, the "endless" bonus game. Unless Start+Select is enabled, the only way out of it is to reset the game.
The main song played where you select the chapters from said game. An optimized version is included, where changes most of the volume fades for ADSR, deletes some similar notes on some channels and nukes channel 7 (#6) and a single Hi-hat sample that plays once in the song.