Hi, I'm one of the creator the game.
There is indeed an algorithm for the sounds when you select pieces, it's awesome that you noticed it 😄
Our goal was to give a satisfying feeling when players select several pieces. I got inspired by the Zelda games where there is scale when you chain attacks on enemies. It is well explained in this video :
So first our awesome sound designer exported keys based on the main music's scale. I made a small code piece to play those keys one after the other when shapes are selected.Then there was a second step: we realized that it sounded bad when several shapes where selected at the same time or very fast. To fix it I added a code that puts sounds in a queue, then the queue release queue every fraction of seconds to make it sound more harmonious.
I hope I answered all your questions, feel free to ask if you want more details 🙂