Imo, this is how I, myself, would make up the sound type.
Sound Type:
Weak to: Bug, Ice, and Flying(x2)
Resisted by: Grass, Ice, and Bug.(x.5)
Immune to: Nothing(x0)
Resists: Electric, Ghost, and Steel.(x.5, defensively)
Now along with this, I would also make a few altercations to other types, so it balances out:
Water would be weak to Poison
Bug would resist Psychic
Steel resistances date before 6th gen(meaning it resists Dark and Ghost once more)
Electric is weak to Ice.
Okay let me explain why I made the changes I made(in my opinion, of course)
1. Water needs to be nerfed, it only has two weaknesses, and a Water/Sound, would be cripled. Plus Dual water typings with little to no weaknesses(Water/Ground for example) either loses a resistance or gains a weakness.
2. Bug needed a buff. Resisting Psychic, means it can easily counter Psychic types. Although it Psychic has a large weakness to Dark(as they can't hit them), they usually have moves, like Signal Beam or Focus Blast, which could easily counter. And the frail nature of Dark types, means they can't hit them, and Psychic is neutral to Ghost, so another weakness won't hurt them.
3. Also Ice needs a buff. It only resists itself, so another resistance, gives them an upper hand too. Taking out Electric as well, is a nice step in the right dircetion. And i know, this would mean, they counter 6 types, but Ice types are usually frail and easily dealt with. Especially with fire types being common.
4. Electric and Ghost needs a nerf. Electric has one weakness. And an Air Balloon, or Levitate(im looking at you Electrik) negates that one weakness(although not long with Air Balloon) Ghost needs a MAJOR nerf. They are usually VERY bulky(Sableye/Dusclops) or VERY offensive(Gengar/Chandelure) so giving them something to look out for, since sound moves are POWERFUL, wont be bad.
5. With Steel, its obvious. Since it resists two types again AND is strong against Fairy, its better that it gets that weakness.
And plus it all kind of makes sense :)
Weak to Bug: Bug uses vibrations and sounds to communicate
Weak to Ice: Sound travels slower in colder environment
Weak to Flying: Sound travels slowest in higher elevations(so the air :p)
Strong against Steel: Sound travels fastest in Solid objects
Strong against Electric: Sound is created through processes of Electronics. Ik it sounds backwards, but its a reason.
Strong against Ghost: People use an array of tactics to track Ghost, one of them, is trying to personally communicate to them, or track traces of Sound.
Resisted by Grass: Actually, idk. Just there to buff grass a bit.
Of course, these are my opinions, but its a suggestion.