Finding all the little sounds was fun! Each one was cute or surprising. I think with a little more particles and sound balance, it will make it more obvious when you've collected a new sound (and show how many are left to collect).
The song building was very silly with how free form it was, which I liked. I think the buttons you spawn don't need to make sound every time you mouse over them -- it got a bit overwhelming when making a song and just moving sounds into position triggered a bunch of them. I also wish that the song I made played in the ending scene!