Level select:
Hold A and press Start, select a level and hold X + Start at the title screen.

Options menu:
Hold A and press Start at the title screen.

Sound test:
Hold X + B + Up/Right and press Start at the title screen.

Beast select:
Hold X + A + B + Down/Left and press Start at the title screen.

Continue game:
When you've died, hold X and tap Start until restarting.

Two-player mode:
Press Start on controller two to begin game play.