On a recent Monday, six people — soon enough four, then two — were browsing the bins of compact discs at Norman’s Sound and Vision, a music store on Cooper Square in Manhattan, around 6 p.m., a time that once constituted the daily rush hour.A decade ago, the number of shoppers might have been 20 or 30, said Norman Isaacs, the owner. Six people? He would have had that many working in the store. “I used to make more in a day than I probably make in a week now,” said the shaven-headed Mr. Isaacs, 59, whose largely empty aisles...