Pokémon X and Pokémon Y

Sound type wouldn't work for a variety of reasons, like certain moves such as Bug Buzz.

Bug Buzz is a Bug Type move that has the Sound property (blocked by Soundproof). If it was given Sound as a type, it wouldn't be a Bug type anymore and a whole bunch of pokemon lose out on a STAB move.

New types wouldn't work in general because of the fact that they would have to not only add new types into the mix of, what, 600+ Pokemon? But they would also have to possibly change types of pre-existing Pokemon as well as moves, as well as possibly changing move sets in general.

I tell my girlfriend that they basically missed the boat when it comes to more types in Pokemon. If they were going to add more then they should have YEARS ago, possibly along with Gen 2's addition of Steel and Dark.