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Sound Test is a part of Mario Sports Superstars that is always open, though some songs are locked until the condition to unlock them is done. It is accessed by going into "Collection" from the main menu then tapping the "Sound Test" button. This will bring up a list of songs which are mentioned below. Additionally, music doesn't play automatically, though there are songs that loop.
The game's music is composed by Motoi Sakuraba. Unlike Super Smash Bros. for Nintendo 3DS and Mario & Luigi: Paper Jam, music doesn't play when the system is in sleep mode. Additionally, even if the background music is set to off, players can still listen to music in the Sound Test. Unlike most games with a Sound Test option, this one has a few pieces missing.
List of Songs in the Sound Test
Note: * means that it is unlocked after completing Star Cup in the respected sport.