The music is easily the highlight of this, I love it a lot! It's super clean, and it sounds lovely.
The gameplay has a few bits that you already know about that can be improved, but I mostly think that the crux of the first 4 levels is too much similar - A bit of variety would've helped.
That said, I super love the mechanic at level 5! I never see people implement UI mouse scrollbars in gameplay, so it's neat :o
Keep on fighting,
Blawnode.