Thanks for the in-depth review, and the kind words !
I honestly don't know anything about audio, other than how to make it play. :-) I never noticed that separation the way you did though. I'll take another look at that.
Yes, the invisible grid is an issue. I probably should have added some kind of tile/table pattern, just to make things more obvious. But at the very end I was more focused on adding more interesting challenges while I still had the time.