(Willis). Now available with online audio backing tracks, this classic series from Willis Music encourages accuracy, provides motivation to learn quickly, raises the standard of musicianship, and introduces a true sense of fun into learning. Books feature Bravo Box reward stickers. Audio is accessed online using the unique code inside the book and can be streamed or downloaded. The audio files include PLAYBACK+, a multi-functional audio player that allows you to slow down audio without changing pitch, set loop points, change keys, and pan left or right.