One of the most extraordinary works to come out of Vienna in recent years, Klaus Lang’s einfalt. stille. (“simplicity. quiet.”), offers a mesmerizing hour-long venture through cloudscapes of sound created by flute, female voice, viola, and percussion. The performance features a rare appearance by the astonishing Russian singer Natalia Pschenitschnikova. This is music through which we drift, or that drifts through us. “Through concentration on listening,” Lang writes, “we can enter a state of simplicity of mind which is a state of the highest inner clarity or inner silence.”