The sound effects are a step in the right direction! Add some visual feedback on click to make the game feel more alive. For example, when you click a circle, it can jitter before disappearing, or change size and pop. Think creatively about how you communicate responsiveness to your players.
Have you thought about making this for mobile? The touchscreen controls would work well here, and perhaps you could adapt it so that rather than clicking, you can drag and draw a line around the balls to create shapes, which reward the player with a nice visual update on completion