What a nice way to make a marble course more interesting! I really like the concept. It feels quite satisfying to play, though I think it might enhance the experience if the sounds of the notes would be a bit less electronic and more soothing, in the same style of the background music. I think then it would enhance the peaceful feeling. Maybe the icons on the top right could be a bit bigger as well, so that it migt be more intuitive to the user to grab those. For me it was a bit unclear until I saw the description, and maybe making them a tad bit bigger would make it obvious enough right away.
But those are only small things I noticed, overall I really like it, well done!