Hidden Mode 1Hold L + R (the two back keys on the Beat Mania controller) and press Start when the menu with the "Press Start Button" selection appears. Release those buttons and press Left + X (the two white keys on the left and right side). If entered correctly, you will hear a sound.
Double Play ModeHold Left + Square + X (the three white keys) and press Start when the menu with the "Press Start Button" selection appears. Release those buttons and press L + R (the two black keys). If entered correctly, you will hear a sound.
