On the title screen press start and pick the option "SOUND TEST". When you're in the sound test screen press X+Y+Z and a little piano will key out the tunes.
Gold Badges
Beat a parallel and wait until the credits have completed. Answer "Yes" to save your game. A gold roman numeral badge for the parallel that was beaten will be displayed.
Mine Cart Views
During the mine cart race in parallel 2, press R to switch to a first person view. Press L to return to normal.
Two-Player Mode
Beat parallel 5, and enter it again. The fighting tournament will now support two players.