With a potion door ready, grab a Starman, then quickly enter Sub-Space. Stay there until your time is up, and your invincibility should be ending just as you are sent back to the normal level. Instead of the normal background music returning, the Sub-Space music will keep playing over and over.

Note: Though each Sub-Space visit only lasts for about 6 and a half seconds, the Sub-Space music data stored in the game is about 14 seconds long. Using the glitch above, you'll get to hear the entire 14-second remix of the Super Mario Bros. theme.