Press the following at the title screen, or by pausing the game for the desired affect. Some codes only affect certain minigames.

Effect	Code
Hold A to freeze background	Left, Down, A, A, B, Left, Up, Down
Invincible	Left, Right, Down, Down, Up, C, A, B
Secret Sound Test Menu	Up, Up, A, C, Down, Down, Left, B
Unknown Effect	Up, Down, Up, Down, Left, Down