I really loved the artwork for your game, the style is super cool! I think the initial idea of your game with extending a note would have been pretty creative and brought the game to the next level. As of now, the countdown doesn't really add any new mechanic to the game, it just represents when the game is going to end. Also near the end (roughly half or two thirds of the way through) only my drums guy had notes left, and at the very end I had no notes to hit at all.
It would also be cool if we got some sort of interactive feedback for messing up - I didn't really feel any punishment from messing up besides my score (which I didn't have anything to compare to). Maybe add a screechy note or play the sound effects at the time I pressed it?
That being said, the game itself is a simple rhythm game thats fun to look at! Nice work!