The Stereo is a Common Barricade in Unturned 3. It was added in the 126.96.36.199 update. The ID is 1466. The stereo allows users to play music.
Currently, the Unturned theme song is the only official soundtrack for it (more may be added in the future), but users can upload their own custom music by exporting audioclips from Unity into an assetbundle ending with .content. Then create a stereo song asset from the asset browser (right click) and drag/drop the audioclip into the song slot. The theme song example file can be found in Bundles/Assets/Songs.