Sound Test:
At game over screen press A,B, and Select at the same time

Music Test:
Before you turn on the game, press and hold down the b button,
then turn on the game. You'll be able to listen to 5 songs.

Reverse Mode:
Hold A and B on controller two and press Start on controller one

Continue:
When your game is over and you're at the blue After Burner screen,
press Left, Left, Left, B, B, B, A, A, A, Start. You will continue
on the level you just got a game over on.