Sound test:
Highlight the "Erase Game" option on the game selection screen.
Press Down, A, R, B, Y, Down, A, Y. Press Select to change the
current song. 

Bonus games practice:
Press Down, Y, Down(2), Y before the game selection screen
appears. 

Easy continue:
With one life remaining, enter a level that was previously
completed and lose your life. Press any button at the game over
screen to return to the Cranky Kong opening screen. Enable the
"Bonus games practice" code and play the bonus games to earn a
surplus of lives. Then, press Start to pause game play, and press
Select to return to the place where the last life was lost. 

Fifty lives:
Highlight the "Erase Game" option on the game selection screen.
Press B, A, Right(2), A, L. 

Interactive two-player game:
Highlight the "Erase Game" option on the game selection screen.
Press B, A, Down, B, Up, Down(2), Y, A and start a two-player game.
The inactive player can press Select at any time to assume control. 

Hint: Mine Cart level:
Jump over the edge on the right and hold Left until you hear a bang.
Jump over one enemy to complete the level.