Let me say, firstly, that I loved the idea and had some fun playing. As many have done, I will post my opinions on the project below in the hopes that it will help you steer the game in the best direction. I wish you the best of luck!
There is something very professional-looking in this game. Something about the layout and the fonts in the menus.
The concept is awesome and it is realized with relative success.
Good choice of colors and pleasant sounds(except my own).
Not having the gift of music, I found that I constantly made mistakes with my voice. This actually speaks of how well your game perceives notes and encourages people to tune themselves better, but I think the difficulty curve should be less steep in order to hook people up for longer.
I understand there is a choice of style in the art. That it is simplistic and focus on colors. This choice is good. Regardless, the drawings themselves are just not good. They look strange at their best and straight lazy at their worst. The mass of enemies that move together is just ugly.