Enter the Options menu then quicky press Up, Down, C, Down, B, B, B, Up, C in order. Return to the main menu and then enter the sound test menu. Return to the main menu again and re-enter Options to unlock the ability to adjust the number of Credits you can start the game with.
To do this trick, you must have enough energy to do your special move. First you must be the robot, Scooter. When you die, use its special attack move and it will end up with no energy, but it will not be dead. Do this trick every time you die, and (if you are good enough to power up your life again) you will not lose any lives.