The 1920s glamor of the Belasco Theater in downtown Los Angeles provided a stunning setting for a triumphant performance by Mumford & Sons for an episode of Live from the Artists Den. The band delivered songs from its Number One album, Babel, along with such hits as “Little Lion Man” and “I Will Wait,” before closing the evening with a blistering rendition of “The Cave” from the classic, multi-platinum debut Sigh No More.