During the game, on Controller 2, press A to cycle through the 4 different background melodies, and press B to toggle music off and on.
Input the 2nd tile in the 2nd row from the top (2 chips) as the first spot, then the character for "middle" at the left of END for the second spot. Press END around 20 times to get the Ending started.
When you finish the game, leave the Ending sequence running to return back to the main menu. Start a New Game or Continue with the 2nd option to access an additional 40 stages including more story.
Use the password to select a round but have no score. Think of the password screen as an (x,y) grid to select the tiles. You must press END five times to confirm.
Use a combination of two top row numbers (1 through 9 in Chinese/Japanese characters, 0 is the first character in the 2nd row) to warp to the desired round. For example, 9 9 will go to Round 99. Press END to confirm.