Koji Kondo joined Nintendo in 1984 and ended up composing basically every memorable theme from the Mario or Zelda series. The legendary composer had a particularly wild idea for a stage effect in last year's Super Mario Bros Wonder, and while that idea was rejected, it did end up giving Kondo the opportunity to bring his voice to the game.
Wonder director Shiro Mouri detailed a few unused level concepts for Super Mario Bros Wonder in a recent Game Developers Conference Talk, including "an idea from our sound director, Mr. Kondo. His idea was 'when a Wonder effect activates, a live-action version of Mario with human proportions appears, progressing through the stage while humming the background music and mimicking the various sound effects himself.'"
