Hold 1 + 2 + 3 + 7 + 9 + Option when the "Eye" appears on the title screen.
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.
Set sound and music options
Pause game play and press A, B, or C.
Subway Passage bonus level
Press * at the background selection screen.
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.
Voice modem mode
Press 9, 1, 1 at the title screen. The word "Awesome" will confirm correct code entry.
Bonus options...