Ninja Gaiden 2: The Dark Sword Of Chaos

Sound tests:
Hold Up/Left + Select + A + B and press Start at the title screen to display Ryu and allow all music from the game to be played.

Alternatively, press Start after the title screen fades to display it again. Press Up/Left + A + B +Start to display Irene Lew and allow all music from the game to be played.

Finally, press Start after the title screen fades to display it again. Allow the screen to fade again and press Start. Press Up/Left + A + B +Start to display Ryu and Irene Lew and allow all music from the game to be played with a volume and song title display.

Invincibility:
Quickly press Up(3), Down(3), A, B, A, B, Select, Start at the title screen. A sound will confirm this part of the code. Then, press Right, Left, Right, Left, B, A. The word "Invincible" will appear to confirm correct code entry.

Unlimited lives:
Begin game play on level 4-2. Jump up and intentionally get hit by an opponent to land on the first ledge of the screen. Collect the extra life, return to the ladder, and climb up to the screen where your character was hit. Repeat this process to collect as many lives as needed.

Hint: Power-up items:
Information in this section was contributed by Jason Hogan.

Red Ninja Symbol: Allows the Ninja to split his body into a twin; limited to two twins.
Blue Ninja Symbol: Gain an extra player.
Medicine: Restores strength.
Bonus Bags: Gain an extra 1000 to 2000 points.
Scroll: Increases ninja power to the maximum.

Hint: Power-up weapons:
Note: Collect as many of the Ninja Power Symbols as possible, as they are needed to use special weapons.
Information in this section was contributed by Jason Hogan.

Throwing Star: Basic throwing weapon that uses five power points.
Windmill Throwing Star: More powerful than the basic throwing star. Will return to the direction thrown, uses ten power points.
Art of Fire Wheel: Deadly fire weapon that uses eight power points.
Fire Dragon Ball: You must possess the Dragon Sword to convert your attack into electrical power, uses eight power points.
Invincible Fire Wheel: Rotates around the ninja's body making him temporarily invincible.

Hint: Using power-up weapons:
To use a power-up weapon (for example, the Ninja Star) hold Up then press B.
Information in this section was contributed by D. Gosse.

Hint: Wall jump:
Press A while pressing the D-pad in the opposite direction you are facing off the wall.
Information in this section was contributed by Jason Hogan.