Psycho World (MSX2)
This game has a hidden sound test.
Psycho World is a technically impressive side-scroller for the MSX2 featuring smooth horizontal scrolling, something the base platform didn't natively support. This game is also what the later Sega Master System and Game Gear ports would be based on, renamed Psychic World there.
When booting up either disk, hold M before or on the "Loading PSYCHO WORLD..." screen. This will open the sound test for the game.
Use the arrow keys to navigate the sound test, and press Space or the Shift key to select the options. Choose えんそうする to play the track, and ちゅうだんする to stop the music. Selecting ゲームをはじめる at the end of the list will start the game.