When you've successfully beaten the game, wait for a Congratulations screen to appear. It will also show the Kemco logo and copyright. Hold down L and R together until the game returns you to the main menu. The sound test will be under a new option, titled ???
Stage Select
To use the code, reset the game and wait for the Title screen to appear. You must wait about three minutes for the Title screen to come up on its own. Hold L and R on both controllers, then press the appropriate buttons below to start on a specific stage:
Stage 2 - Simultaneously press A on both controllers
Stage 3 - Simultaneously press B on both controllers
Stage 4 - Simultaneously press X on both controllers
Stage 5 - Simultaneously press Y on both controllers