Press any direction on the D-pad, followed by A, B, X, or Y at the Nickelodeon screen. The sound that is played will vary, depending on which D-pad direction and button were pressed.
Quickly enter the following code at the title screen. Hold R + Up and press A(7), hold L + Up and press A(8), hold L + R + Up and press B(9), then hold Up and press X. You will be taken to a screen with Oblina to confirm correct code entry. Press Left or Right to cycle through the levels, then press Start to play the selected level.
Press L to enable invincibility and R to disable it at the level selection screen.
Make Grumble blink:
Press L or R on the screen that appears before the start of a level.
Make Grumble open his eyes wide:
Hold R + Down when Grumble appears before a level begins or at the options screen. After releasing the buttons, Grumble will appear to be angry.