WarioWare Inc: Mega Microgame$

Super hard mode
Successfully complete the game on the hard difficulty setting. Start a new game, and the super hard difficulty setting can now been selected.

Listen to CDs
Whenever you get a CD during gameplay, it will be available to listen to later. Once you get one, go to the Sound Room to hear it.

Unlock Wario Karaoke
To unlock Wario Karaoke, go to the sound room. Highlight "Exit," then press R + L + SELECT + START + UP.
Once it's unlocked, you can do different things to the CDs you've collected. You can speed them up or slow them down using LEFT and RIGHT. You can change the pitch of the song by pressing UP or DOWN. Or, you can toggle between different colored CDs by pressing L or R.