Re: Sound type
Sound would need to fuck with as few vulnerable types as possible while also helping out some weak ones, even if it means making some matchups inert that would otherwise seem like they need some interaction. For example, it makes sense for Water to resist it, but it isn't really necessary balance-wise and just makes Water even better defensively than it already is. Likewise, it makes sense for it to be SA against Bug, but that type doesn't really need any further weaknesses.
These sound alright to me:
x2 attack against: Sound, Fairy,
x2 damage received from: Sound, Electric
.5x attack against: Ice, Ground, Psychic
.5x damage received from: Steel, Flying
But even so, there's so much that goes into introducing a new type into the series, then there's the fact that certain candidates for re-typing would encounter situations in which they lose something by dropping the old type. For example, how much does a Pokemon like Noivern gain from losing either Dragon or Flying to become part Sound? Also, the number of moves that could be reclassified as Sound is comparatively small, so they'd have to come up with a lot more moves for it.
I'm not comping at the bit for a new type, but if there were to be one, Sound is the one that makes the most sense to me. But it brings with it a nightmare of logistics and balance changes.
