Thanks for the feedback. I am not trying to feign ignorance, and I would like to find a way to properly license the content and compensate the rights holders.

Thanks for the feedback. I am not trying to feign ignorance, and I would like to find a way to properly license the content and compensate the rights holders. However, I've looked into it and I don't currently think there is a license type (mechanical/synchronization) that is appropriate for these apps. The apps do not contain any recordings and do not offer un-aided playback of the "songs." The user has to tap out his/her own interpretation of the notes, to make it sound like the song.