Ruth Ann Swenson is a highly accomplished and enjoyable singer, but not an all-purpose one. Her rich, full tone, accurate coloratura, easy high notes and beautiful trill work wonderfully in "Rigoletto" and in "La Sonnambula," for example; she seems dramatically committed and engaged in those roles, too. Here, she seems detached, clueless and stylistically at sea -- neither her tone nor her phrasing conveys the luscious sensuousness of Semele's arias, and her Mozart is similarly bland and uncomprehending. It's surprising that Charles Mackerras, usually a superb and enlivening conductor of both these composers' music, could not stimulate or lead her to something better.