I've mined Mutopia for the majority of music, but there are maybe 30 songs I've done by hand just because they were obvious holes in the selection. I'm a little unsure about offering CC-licensed stuff in the app, but my Etude-converted songs are also offered under CC and I'm going to be linking to the licenses, so I think it should be fine.

I'm going to try to write a post to show HN about the tech behind it. I wrote my own .ly parser/preprocessor in Python as part of the song conversion. To help the forked version of Lilypond correlate the screen position and time offset of notes in the piece, I ended up having it do things like unroll macros and loops and do all the layout changes I needed to make em work in the app.