Much of Bach’s chamber music has been lost. But what we have are real gems. His music for violin and for flute, both accompanied and unaccompanied, are central to those instruments’ repertoire. His music for viola da gamba and harpsichord is also unique and of the highest quality. This program, recorded at the Mahaiwe, features a sonata for violin and continuo, a duo sonata for flute and harpsichord, and excerpts from one of his late contrapuntal masterpieces — A Musical Offering.