Mettaton is an incredibly flamboyant robot who only wants to perform on stage for humans. His goal is to acquire the human's SOUL and go through the barrier himself, so he can finally achieve his long-awaited point of stardom. Any voice works for him, really; just sound loud and flamboyant, and you're good. However, I would prefer it if you added an effect onto your audio so you sound like a robot; this isn't required, though!