Glad you like it! A couple of people have asked for this feature and I guess I've been struggling with how to fit this concept into the game.
Maybe the answer really is just as simple as a little visual toggle that puts you in a "sandbox" mode where all the sounds still go through, but the game doesn’t respond to them until you untoggle it.
Another approach is having an "explore and test" or "Training Wheels" mode. Correct notes played by the user behave exactly as they do right now. But for a wrong note played by the user, instead of the round ending, you could just gives some audio-visual feedback that this is not the right answer but continue to expect the user to find the right answer. This way we can hunt and peck our way to the end of the round.
This creates a problem in that it's easy to muddle your way through without learning anything. To prevent this:
Once it gets to the end of the round in this mode, if the user had even 1 wrong note selected in this round, the game will then expect them to play it perfectly once again (like it does now).
This way you get both the hunt and peck exploration and the final "now that you've had your time to get your ears and fingers in order, play it correctly."
Put me up as another person wanting that feature, it was how I expected it would work.
same!