Grab the flagpole while invincible with Starman power, and the overworld theme will interrupt the "course clear fanfare" and continue to play until the game moves to the next level. The ideal level to try this in is World 1-1. Hit the block with the Starman in it, have the Starman follow Mario up to the first small staircase, then grab it and head through the rest of the level as fast as possible.