Loud noises give headaches so if anything psychic would be WEAK against sound, logically speaking.
Pretty much every culture at one point or another has used music/chants/prayers etc to drive away evil spirits and having sound being SE against/resist ghost could play off that.
Sound travels through liquids and solids(especially metal) faster than through air so maybe have flying resist it while giving steel and water a weakness?
Let ice be SE against it because you know, it's hard to speak when you have a cold. Poison could have a similar argument since it makes the victim ill(sore throats during colds are usually caused by bacteria). Fighting attacks could knock the wind out of sound types, making it difficult for them to use their attacks so that could work as a weakness too, although giving fighting another buff is iffy.
There's a little myth that singing to plants make them grow, so grass resisting it would give it a much needed resistance. Bugs could use a buff so why not give them a resistance to it as well(i mean with all the buzzing they do they're probably well attuned to loud constant noise.) I can see Fairy getting another resistance via sound, or even normal since sound is the most used form of communication.
Echolocation relies on using sound to find things in the dark so a dark type would have a hard time sneaking up on or hiding from them. Also having mixing noise with another noise just makes more noise so it could resist itself too.
How do electromagnetic waves interact with sound waves? I believe a relationship could be formed there too.
Sound can definitely work as a type, just be creative with it.
I really, really, really don't like the idea of a Sound type, but there is at least some precedence for it. I don't really see how it would work, and feel like it would be an extremely niche type, but I probably said that about Fairy before the games were out.
Light is basically Fairy type, and Air... I like the idea of an Air type less than I do of a Sound type.
I just cannot see another type other then sound honestly. What has not been covered in some form other then Sound. And I think plasma is technically made up of things that are present in the other types, like electricity so I would rule that out.
" Don't curse the darkness--light a candle! When freaky aliens give you lemons, make freaky alien lemonade. "
Super Effective Against: Fairy, Fighting, Ghost Less Effective Against: Ice, Water, Electric, Dark Weak to: Electric, Ice, Dark, Electric Resists: Fighting, Ghost, Fairy, Sound, Water
And no, I didn't make a mistake with water. Sound causes ripples in water, but the water is ultimately unaffected. Meanwhile, sound travels better in water so water would aid a sound type more than harm it... As such, both resist the other.
On the other hand, Water type Pokemon are not Water itself. The sound attack would be more disruptive to Pokemon underwater than Pokemon on land, therefore Water is weak to it. It's similar to the reason I think Steel should be weak to Electric, the metal parts of the Steel Pokemon conduct electricity to their non-metal parts making them more vulnerable to it.
And more importantly it makes more sense balance wise.
I also don't think Ice makes sense (seems like it would be the opposite if anything because of avalanches) but it's very necessary balance wise.
Sound would only be retyping a few pokemon, like the whismr family, jigglypuff family, chatot, meloetta and... That's about it... So it's pretty useless unless Z was gen 7, which is not likely. Light = Fairy Air = Flying And sound kinda sounds like a mix of normalxsteel anyway... And a LOT of normal type moves such as growl, sing, hyper voice and stuff will have to be re-typed. So I don't see the point in having a new type, unless the new type weakens the fairy type making it less broken.
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Resistant to - Fire, Water, Grass, Electric, Flying, Steel, Fighting, Rock, Ground, Poison, Psychic, Dark, Bug, Dragon, Normal, Ice, and Ghost
Immune to - Fairy
And as an added bonus, all attacks with the Dragon's Revenge typing are "Double OHKO's" to all Fairy types. It kills them once, heals them to full health, then kills them again. ALSO, they all have +9 Priority and Infiltrator and Feint built into them.