Turbo:
Hold 1 + 5 + 9 at the title screen. The word "Fight" will confirm correct code entry. A turbo option will now be accessible from the main menu.

Bonus options:
Hold 1 + 2 + 3 + 7 + 9 + Option when the "Eye" appears on the title screen.

Voice modem mode:
Press 9, 1, 1 at the title screen. The word "Awesome" will confirm correct code entry.

Fight against Carbon:
Begin a game under "Hard" or "Killer" difficulty levels. Win the "Earthquake Zone" stage with a double flawless victory. This may be easily done by defeating the first opponent in the game, then intentionally lose and continue all other matches until the "Earthquake Zone" appears. Then, hit your opponent once, then block until the time runs out.l Carbon will appear in the "Subway Passage" stage.

Hidden Palace bonus level:
Press # at the background selection screen.

Subway Passage bonus level:
Press * at the background selection screen.

Set sound and music options:
Pause your game and press A, B, or C.