In the studio, Led Zeppelin were masons and their songs were chiseled megaliths. But onstage they treated sound like clay, molding and remolding it night after night. The soundtrack to the icons’ 1976 concert film documents them at their most exploratory. Most tunes, especially “Whole Lotta Love” and “Dazed and Confused,” are stretched to psychedelic infinity—swirling cries, writhing feedback, clattering breakdowns, and funk-blues beats twisted every which way by drummer John Bonham. Getting lost in Zep’s loose, gooey freedom makes for one awesome trip.