At the high score table, before the title screen reappears, press Left 64-67 times. Hold Select after you select a level, then press any of A, B, or Start to start a new game. The bonus probability is boosted by thirty times.
Sound Test
Get 1st place in terms of both score and level, then enter the initials SDT. Press Start, then at the level select menu, select Level 2 and press A, B, or Start.
View Credits
Get 2nd place in terms of both level and score. Enter the initials STF for your name, then press Start. At the level select menu, press Left 29 times, then A, B, or Start.