The title, actually the subtitle, was something like "OR, The Things in [girl's name, I think it might have been Susan]'s Piano."  It was about a girl who was bad at playing the piano, maybe willfully from refusing to practice, and inside the piano were monsters who were hurt or injured or horrified or angry or something, by her bad playing.  They may have been musical notes come to life.  It was a picture book in my elementary school library, so it was published before 1982.  Probably in the 70s.  It was sort of psychedelic and the characters were almost ugly, deliberately so.  I remember lots of teeth in big mouths.