Music test:
Press J + B at the title screen.

Edit levels:
Note: This procedure involves editing a game file; create a backup copy of the file before proceeding. Use a text editor to edit a level file. Enter the following characters to change that level.

X: Player one starting position
Y: Player two starting position
2: Destructible blocks
0: Space
1: Indesctructible blocks
3: Coins
A, B, C…: Various enemies