The Cassette is one of the salvage items found in the first Endless Ocean. It's located at co-ordinates F-4, being one of the two treasures, the others being the Ancient Bracelet, inside the Gallery of Souls. This treasure is automatically accessible as soon as the player has unlocked the Rock Bluff area (where the Gallery of Souls is located), unlike the Bracelet, which must be unlocked through a fair amount of investigative archaeological work in Mo'ia Atoll.
In your footlocker aboard the Gabbiano, the Cassette is described: "A magnetic tape in remarkably good shape, despite its being at the bottom of the ocean. It might even play!"