I do sometimes wonder if our understanding of types is also based in part on their having both an attack and pokemon component. Is that why Sound isn’t a type, for example? It has “attacks” in that some pokemon are immune to sound-based moves as a category, but there are no known pokemon that exhibit different weaknesses and resistances that would classify the pokemon themselves as Sound. Am I close?

That’s certainly part of it, @emeraldmew. Putting it another way, all attacks are also sorted based on elemental resonance, so what they are tends to be clearer cut than a pokémon’s attributes. Or, rather, they were after we redid the classification system in the late 90s. Before then, pokémonologists considered certain moves to be types they most certainly weren’t. Gust, for example, had been a normal-type move, and Bite was normal until we created the dark-type classification. In recent years, the only moves that had been subject to reclassification were non-damaging fairy techniques such as Sweet Kiss, and that was partly due to the fact that it’s harder to determine the elemental resonance of something that doesn’t do damage to a pokémon and partly because we had only recently created the fairy-type designation.

Nonetheless, the point is, techniques all have elemental resonances, and the way these elements interact with a pokémon’s physical attributes (that is to say, their body, not simply what one might consider “physical defensive abilities” or “defense”) is how we categorize types. While sound-based moves such as Hyper Voice and Uproar have a consistent pattern of use, the problem lies in the fact that not all sound-based moves resonate with the same element. For example, Hyper Voice is considered to be a normal-type move because it behaves as a normal-type technique would. Bug Buzz, meanwhile, integrates an insectoid element into its use, which is why it’s super effective against types Hyper Voice is only neutral to. This is true with every sound-based technique, which leads researchers to believe that there is no move that has a “sound resonance.”

But it’s really more than that as well. Just as we’ve observed (so far) that there isn’t a sound element, we’re able to observe the way other, established elements interact with pokémon. For example, take swirlix, a pokémon who is classified as fairy. The way type research works is scientists expose each pokémon to the eighteen known elements and record their reactions. Likewise, we record data concerning the way the subject uses each element as well. Combining both sets of data, we’re able to cross-reference and see not only what the pokémon failed to or was able to resist but also which elements the pokémon wielded best. Any discrepancies point towards the possibility of a new type.

Swirlix was among the first pokémon to be officially registered as fairy, and as such, it was also one of the reasons why scientists went back to reclassify pokémon that had already been entered in to the National Dex. Originally, it was proposed that swirlix was a normal-type and that many of its natural techniques—namely Fairy Wind, Play Rough, and Draining Kiss—were either normal or flying. However, by observing not only the way the dragon type interacted with swirlix and the way swirlix was more adept at using those three aforementioned moves than the actual normal techniques it knows, it was determined that swirlix and its basic movepool may not be a normal-type at all.

In short, you’re more than close: moves are part of the pokémon classification system. They’re part of a very complex taxonomy that calls upon a number of different variables centered around the way pokémon interact with them and vice-versa. For that reason, though, it can be difficult to recognize when a type exists, but as sound-based moves do not produce consistent results among sets of pokémon, it’s very unlikely that a sound-type exists.

Or in even shorter terms … yes.