To hear remixes of the game's music, go to the BGM Test (Background Music Test), play any track, and press R to make the music fade out. While it's fading, continue pressing R as you hear musical tones played consecutively. Eventually, you'll hear a remixed version of your favorite tune.
At the title screen press Down, R, Right, Down, Right, Right, Down, Right, Down, Down. Then pause the game and press R, Right, Down, Y, Down, Right, Down, Left, Right, Down, Right, Right.
Press R = Shotgun Bomb
A = Sky Laser
B = Ground Laser
X = Reflect Laser
Y = Spread Laser
Then press A = Homing Missile
X = Falling Bomb