>>32640093"Sparkling Aria - The user bursts into song, emitting many bubbles."
These bubbles, charged with sound, are controlled by Primarina's voice and explode when near the enemy. That's why it is a sound move.
Popplio is shown to be a hard worker that diligently practices on making bubbles in the anime and almost every description: "Popplio can create balloons made of water from its nose and utilize them to create a variety of different strategies and attacks in battle. This Pokémon is better at moving in the water than on land, and can swim at speeds over 25 mph. On land, it uses the elasticity of the balloons it creates to perform jumps and acrobatic stunts."
Brionne is said to make bubbles through dancing: "This Pokémon is a hard worker and pours itself into its efforts until it has memorized each dance. As it dances, Brionne creates balloon after balloon. In battle, it first sends its opponent into disarray with its dancing, and then slaps its balloons into its target, causing the balloons to explode and deal damage."
Primarina does a bit of both, but now can and mainly uses its voice to control bubbles and deal damage.
Sort of unrelated but you can also faintly hear the sound of bubbles during Primarina's cry.
They are not pearls, they are bubbles.
