I love the progress on this. It keeps getting better and better. Some things I noticed about using a background track. With some songs, their volume is so loud that I can't hear anything from when I hit the Pyano notes. It would be great to individually adjust the volume of the notes and the background song.
Another small thing with the background song option. If you have one song playing, and choose another song without telling Pyano to stop playing that song, both songs will play. I kind of like the option, but don't want to eat up all my memory trying it too much.