Steel Type is basically Light, considering moves like Mirror Shot and Flash Cannon who are both based on Light are Steel Type moves, anyway if they make a Sound Type then Whismur, Loudred, Exploud and Chatot would be classed in it, moves like Uproar, Hyper Voice, Chatter, Screech, Howl, Growl, Sing, Perish Song and other moves blocked by Soundproof would be classed as Sound Type moves (apart from Bug Buzz). Also they could be immune to other Sound Type Moves and that way the Whismur line and Chatot would be able to get other abilities!
If they make a ??? Type then Hidden Power who has varied Types, should be one of the moves and Ditto, Kecleon and Arsues (and other Pokemon who can change thier Types) should be part of it.