“For decades, the Tanglewood Festival Chorus, the Boston Symphony Orchestra’s all-volunteer group of singers, dazzled audiences with its performances, delivering masterful renditions of intricate scores in German, Czech, Latin, and French — almost always from memory. But discordant notes are being struck after the chorus’s new conductor, James Burton, has unceremoniously decimated its ranks, forcing out a large swath of singers, including many of the group’s most senior members.”