Thank you for this kind and deep feedback.
I really appreciate that you (and other people) took time to build the cubes and test my game. I'm aware how it was unusually complicated to try it and demanded effort without even knowing so clearly in advance what to expect with it. So thanks a lot!
That's interesting what you said about the difficulty. I was actually making sure to match the tempo for the songs (for both classical and rock) to make it more subtle because I thought it would be too quick to solve if the mismatch was too obvious. Maybe I was too much trained myself while preparing those songs to be aware that it would have been enough difficult without doing that. I should at least add different difficulty levels with the songs prepared in a different way.
And yep, sorry for the small cubes. I made the final pdf at the last moment, and didn't try enough with the final size. I should make them bigger definitely.