The Walkman is 25 years old. Norman Lebrecht says it transformed (and cheapened) music. “Its advantages were many, mostly unforseen. Actors learned their lines by Walkman on the bus into rehearsal. Splenetic executives used it for lunchtime meditation. I once heard Mahler’s Resurrection Symphony on a vertical Alpine train as a thunderstorm crashed all around. In unforgettable settings, music acquired unsuspected dimensions. But these benefits were soon outweighed by its corrosive effects.”