At the title screen, quickly press Right, Down, Down, Down, Right, Down, Down, Right, Down, Right, Right (hold it) and press either I, II or Select. If done right, a sound will be heard. Now start the Endurance mode and you'll see new endurance times added for each track.
Mute The Music
At the title screen, quickly press Right, Down, Down, Down, Right, Down, Down, Right, Down, Right, Right (hold it) and press Run. If done right, a sound will be heard. Now the music will be Off during races.