As getting design feedback is an important part of this website, I'll throw that in of my first impressions from playing:
1) Consider adding keyboard input. Maybe sth like an octave of:
w e - t y u
A S D F G H J2) Take responsibility for your scoring system ;)
If you want to reach a good score …
Do you reeaally want to force people who are bored with lower levels to sit through them to get high score?
Adjust the scoring for maximum fun and minimum "additional necessary hints" as the one above ;).
3*) As there is so much freedom for the player, this game is the closest to the main focus of our wiki. So I'll add that too: maybe it would be fun to make it a bit more creatively involving? You deal with sounds sequence, if players were adding notes to the sequence one by one the game would be… semi-compositional ;)