Supplements A Modern Approach to Classical Guitar. This book contains 32 classical guitar duet arrangements for either recreational or instructional use. The sequence of pieces is based on the difficulty of the first guitar part. However, the level of difficulty is indicated at the beginning of each piece for both parts. The accompanying 45 minutes of audio allows players to practice and play each piece as a duet. The audio is accessed online using the unique code inside each 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.