Everyone is asking, or predicting, “what’s next.” That’s what you do in a pandemic. When now?” And Luciano Berio, applying Mahler’s symphony and Beckett’s novel as framework in his “Sinfonia,” took “What next?” to the next step. Berio took the second movement of Mahler’s Second Symphony, known as the “Resurrection,” and added musical commentaries in the form of snippets of …