Minecraft wiki entry for Development_versions, associated text: Records
Items can now be made records to play music in jukeboxes. Their component variables:
sound_event A string value corresponding to a sound event in the game code. This string must be one these for music to play: "13", "cat", "blocks", "chirp", "far", "mall", "mellohi", "stal", "strad", "ward", "11", "wait", "pigstep"
duration A float value that determines how long particles are spawned from the JukeBox Block, should approximately match length of sound event.
comparator_signal An integer value that represents the strength of the analogue signal, used by the Comparator Block.