Debussy’s Prélude à l’après-midi d’un faune begins with a sigh: a long, languorous exhalation played on the lower notes of a solo flute. The flute’s usual brightness and brilliance is gone. It’s a dusky, breathy sound, made of half-shades and velvet: the musical embodiment of luxe, calme et volupté. And it’s completely impossible to imitate on a piano.