Wario Land 4 (GBA)

Unlock Karaoke mode
In the Sound Room, move the hand pointer to the "exit" button, then hold L + R + Start + Select and press Up. A "karaoke" button will appear next to the exit button. Select the karaoke option, and a new song will play. During the song, press L or R to toggle the vocals, Left and Right to change the speed, and Up and Down to change the pitch.