FYI this has been a big challenge even for music that has no copyright on it. Musopen.org has collected a great deal of music, and even for recordings we bought

FYI this has been a big challenge even for music that has no copyright on it. Musopen.org has collected a great deal of music, and even for recordings we bought the rights to, they automatically challenge it! It works well to scare people from leveraging even public domain resources.