This book constitutes the refereed proceedings and revised selected papers from the 15th International Symposium on Music in the AI Era, CMMR 2021, which took place during November 15-19, 2021 as a virtual event. The 24 full papers included in this book were carefully reviewed and selected from 48 submissions. The papers are grouped in thematical sessions on Music technology in the IA era; Interactive systems for music; Music Information Retrieval and Modeling; and Music and Performance Analysis.