Lower the music volume, and go to a race that features Yoshi spectators in the crowd. They can be found at Donut Plains 3, Sweet Sweet Island, and the Yoshi Circuit in DLC 1. Eventually one of them will hum Totaka's secret eight note song instead of their normal random grunt. Kazumi Totaka's is Nintendo sound designer and has placed the same secret song in other games, such as Mario Paint and Animal Crossing.