We trace back root causes on this week’s show, exploring composers in Italy, who laid the foundations for much of what we enjoy in classical music today. And organbuilders, too, whose instruments in Bologna, Treviso, Turin and Pistoia retained an unparalleled degree of simplicity of design and purity of sound across four centuries of European history. Their unique character blooms in the special idioms of Frescobaldi, Pasquini, Valeri and others, as we discover during A Sojourn in Italy.