The Sound stone is a Key item passed to Ness by Buzz Buzz during the opening events of EarthBound. The Sound stone acts as a kind of recording device, and is able to contain the power of the melodies that Ness and his companions hear at each of the eight "Your Sanctuary" locations. Until it is completed, Ness can use the Sound stone to hear the melody in whatever state it is in at the time.
When the last melody is absorbed by the Sound stone, Ness appears to fall unconscious, yet at this time he is actually venturing into Magicant, a place created purely from Ness's mind and memories.
Upon arriving at the center of Magicant (and defeating the evil and its three guards that reside there), Ness regains consciousness, and the Sound Stone dissappears, as its purpose is fulfilled. This also results in Ness absorbing the power of the eight Sanctuaries, as well as his own inner potential, granting him the power needed to complete his journey.
The Sound Stone can't be sent to Escargo Express.
- Although the Sound Stone normally is a required item, if Ness's inventory is full when he receives the Sound stone, Buzz Buzz instead gives the stone to Tracy, freeing up an inventory spot for the rest of the game since it still records the melodies of the "Your Sanctuary" locations even in Tracy's possession. If the player does wish to have the Sound Stone back from Tracy in this situation, they must talk to her in person as it doesn't show up in the Escargo Express storage.
- The Sound Stone is instrumental to traveling to other places on the map with the "Walk Through Walls" glitch.