Super hard mode:
Beat the game on hard.

Unlock wario karoke.
Go to the sound room and highlight 'exit'. Then press R+L+Up+Select+Start simultaneously. You can now alter the sounds of the CDs you've collected - you can speed them up or slow them down using left and right and you can change the pitch of the sound by pressing up or down. Toggle different colored CDs by pressing L or R.

Thanks to Revolution readers Lucas Chisholm and maxwell!