Sound test:
Hold Left + Select + Start + A + B when the phrase "Tecmo Presents 1989" appears. Continue to hold all buttons until the title screen appears in full. A sound will confirm correct code entry. Press Start to display the sound test screen. Press Up or Down to select a sound, A to play a sound, and B to stop a sound. Note: Hold Down/Left + Select + Start + A + B when the phrase "Tecmo Presents 1989" appears in the European version of the game.

Hint: Shinobi reference:
During the storyline, the game will state that a ninja named "Shinobi" borrowed the Hyabusa Family's Dragon Sword. This is a direct reference to the Tengen game Shinobi.