Hey there. Perhaps it's just me being biased as someone who is a pretty good whistler, but I quite enjoyed playing this game.
The gameplay its self felt pretty good but it was still fairly lacking.
I'd say one rather disappointing thing about the game play is that your notes don't really have durability. What I mean by that is that you don't get much pay off for whistling longer notes. It's nice that your longer notes cover a larger area, but I think it would of been much better that the longer notes also could stand to hit more enemies. It was kind of disappointing that my longer notes would still only result in like one hit and instantly disappear.
In short, I feel that the note length should of effected it's durability allowing you to pierce through multiple enemies.
If that would of made the game to easy, then I'd say the remedy to that would probably be to increase the rate at which the enemies spawned or maybe increased their speed by a bit.
And in terms of the enemies themselves, I'd say that you could of improved them simply by adding more variance to their health and sizes. Like having a large (and thus easier to hit) boss, but with a lot of health.
But I can also understand that you were building this in a time limit, and likely didn't have the time for all of that.