Level select:
Enter sound test mode from the options screen. Play sounds 8, 4, 19, and 71 in that order. The screen will flash to confirm correct code entry. Begin game play, pause, and hold Down + A + C to display the level select screen.

Invincibility:
Press Left(2), A, B, C, Left, Up, Down, A, Right, A at the title screen. A sound will confirm correct code entry.