Highlight the "Exit" selection on the options screen and press [Left, Right(2), Down, Right, Left, Right, Left(2),Right, Down] four times and begin game play.
Sound test
Enable the "View ending sequence" code. Press Left + C during the ending sequence to pause. Press B, C, D, or the D-pad to hear different sounds.
Thirty lives
Press C, Left, Down, B, Right at the main menu.
Unlimited lives
Press Left, Right, Up, Down, Left, Right, Up, Down, Left, Right, Up, Down at the title screen.
Unlimited weapons
Press Up, Left, Down, A, C, D at the player selection screen.