The first two apps do the same thing. Use your phone to take pictures of a song and then it will recognize the music and PLAY IT FOR YOU! You can mute the melody to just hear the accompaniment, you can speed up or slow down the tempo, you can isolate the melody if you are trying to learn the song with clean rhythms. You can also export the midi or musicXML and drop it into garageband to make a great accompaniment track. (you can also change the key, or load the musicXML into sheet music software!) If you already have your song in a pdf, you can just load that instead of scanning the pages individually.