The Sound type is one of the many fanon types from the Pokémon franchise. They are generally an offensive type with some focus on Speed, and general rarity. It is one of the few able to supereffectively hit Normal types, and it's addition to the game removed the Fighting type's previous spot as the only able to do so.
The common Sound-type Pokémon, however, has a few weaknesses. While being able to resist and weaken the Psychic type, it is generally weak to Steel, Rock, and Ground, the later two of which are known to be some of the best offensive types; their attacks won't do anything on a Cosmic type either, unless moves like Foresight or Odor Sleuth are used, or the ability Scrappy is in effect. While its moves are resisted by Steel type Pokémon, a Sound type Pokémon resists Steel type moves as well.
A Sound-type Pokemon, on average, have high Special Attack and Speed, mediocre Sp. Attack and HP, and low Attack and Defense.