Mostly, though, booing at the opera today expresses something quite different: a belief that the art form itself is imperiled by crazily updated stagings. Recently, booing was heard at the Met’s new productions of “Tosca” and “La Sonnambula.” These classic works were given the modern Euro-treatment, and many in the audiences weren’t buying. Nowadays, it’s directors and designers (German critics snidely refer to them as “Kinder mit Konzept”) who are most often booed.