LOVERS of Johann Strauss's music can depend upon an evening of perfect enjoyment James Etherington, Ruth Naylor and Irene Ambrus are singers of operatic style, and Cyril Richajd, debonair. gay and lively, keeps this Viennese operetta on the light side. Peter Graves is a very languid prince and Jay Laurier a mirthful jailer. Those who have known and enjoyed Die Fledermaus have here an opportunity to hear fine singing and orchestration. Richard Tauber is at his best as the energetic Maestro of the best Symphony Orchestra in the London Theatre.