What is the "sound test"?

[Hugh Mungus]

In the upgrade menu you can buy "unlock sound test". What does this do?

[Yanamonie]

Allows you to listen to the music that is in the game. When you unlock it, you can go to the mirror in youre room and select what music you want to listen to

[Onion_Bubs]

Not the mirror on the wall, the one next to the bookshelf.

Also the moment you leave the room, whether to go into a dungeon or just to go downstairs, the music returns to normal.