Attacking sound-based moves, since those are the only ones that would matter for Resistances. Of those, 7/16 of sound-based moves are Normal type, and one of those is the legendary exclusive Relic Song.
Snore, Uproar, Hyper Voice, Echoed Voice, Round, Relic Song, & Boomburst.
Yeah, 7/16, 6 if you don't count the signature move.
In my opinion, Signature Moves should hold slightly less weight, because they may have been designed with a Pokemon species, rather than type in mind; To give STAB or such, rather than accurately representing the type that they are.
But nonetheless, the non-Normal-type Sound-based damaging moves are:
Bug Buzz, Chatter, Clanging Scales, Clangorous Soulblaze, Disarming Voice, Eerie Spell, Overdrive, Snarl, Sparkling Aria.
Of those 9 moves, only 3 aren't Signature Moves. Heck, 1 of them is a Z Move of a Signature Move. & as said, I'm not sure Signature Moves should hold as much weight regarding what's accurate for a type.
1 of the remaining 3, Echoed Voice, also mostly does its damage through Empathic Manipulation, too.
& when you include non-damaging moves, Normal easily has the majority on Sound-based moves, so in my opinion, the type is representative of the ability somewhat consistently.
To summarize the numbers:
Damaging Normal-type Sound Manipulation that isn't a Signature Move: 6
Damaging Normal-type Sound Manipulation that is a Signature Move: 1 (Relic Song)
Non-damaging Normal-type Sound Manipulation that isn't a Signature Move: 10
Non-Normal-type Damaging Sound Manipulation that isn't a Signature Move: 3
Non-Normal-type Damaging Sound Manipulation that is a Signature Move: 6
Non-Normal-type Non-Damaging Sound Manipulation that isn't a Signature Move: 3
Non-Normal-type Non-Damaging Sound Manipulation that is a Signature Move: 1 (Clangorous Soul)
In my opinion, Normal-type moves consistently enough involve Sound Manipulation that it should at least be a Limited or Possible Resistance for Resisting it.
The majority of the time that Sound isn't Normal-type, it's a Signature Move, which in my opinion, lessens how much weight we can assume that move holds for accurately representing its type, when that move was made for specific Pokemon.
While Rotom is made from plasma, yes, that doesn't mean much for resistances imo. The only move that deals with plasma stuff is Zeraora's signature move.
Ah, thank you for clarifying that. I had forgotten about Rotom.