The Music stone (in-game name Kuulokivi) is an Inventory item.
When held, it plays the music from the biome you are currently in. It additionally has a massive gravitational affect on certain celestial bodies.
It is located through and above the Magical Temple (Snowy Depths), inside a floating, diamond-shaped shrine with runic writing. ??? A spell that burrows/blasts through rock is needed to penetrate into the shrine.