To access the Music Test Screen in Seiken Densetsu 3, hold down the Left and Right buttons (not the directional buttons, but the ones on the top edge of the game controller) and then hit the Reset button to restart the system. The Music Test Screen should magically appear!

Note: This also works on some of the emulators. Just hold down the buttons that you have assigned to the Left and Right buttons described above and select reset. Enjoy the music!