Sound Test: Can be accessed with the following button combination with the left D-pad, on the title screen: Up, Up, Up, Down, Down, Down, Down.
Hard mode: To access a harder difficulty level, press the following combination on the title screen using the left D-Pad: 6 x Right, 3 x Left, 6 x Right.
If you did it right, "For Super Players" appears under the "Jack Bros." logo. The differences to normal mode are more and differently placed enemies, different textures in the levels, differently looking boulders and warp tiles, more jack traps and different passwords.
Play as Pixie: To access Pixie as another playable character, type in '4649' on the password screen but confirm the '9' by pressing and keeping pressed L and R Triggers, Select and A simultaneously instead of using the A Button.