Operation C - GameBoy

Enter the following at the Title screen:

Level select   Up, Up, Down, Down, Left, Right, Left, Right, B, A, B, A, Start  

10 lives      Up, Up, Up, Up, Down, Down, Down, Down, Left, Left, Left, Left, Start 
 
Sound test     Up, Down, Left, Right, A, B, Start 

Thanks to Revolution reader Keith Varner!