In addition to hearing beautifully sung choral music at the Vienna Choir Boys' concert Saturday night, the Kennedy Center Concert Hall audience got to see a one-act opera. Mustering up dramatic as well as musical knowhow, the boys performed "By Royal Command," a 35-minute work by 19th-century German composer Konradin Kreutzer.

Kreutzer's music is bright and hummable; his plot is slight and forgettable. The real humor came when some of the boys had to take female parts, donning dresses and wigs and trying to adopt feminine mannerisms and bearing.

Though sung in German, the opera was slightly modified so that critical spoken lines were delivered in English. As a result the audience, which was given a synopsis in the program, could easily follow the story.

Other selections included Verdi's "Vergine Madre," sung with richly blended harmony, and six pieces from Pergolesi's intricate "Stabat Mater," sung with skill and reverence. In a Hungarian song by Kodaly, the 23 boys performed with gusto and rhythmic precision.

Conductor Bernard Steiner, who provided stylish piano accompaniment when called for, led the choir in marvelous, swirling renditions of a polka and a waltz ("1,001 Nights") by Johann Strauss Jr. Steiner also accompanied three of the boys in the infectious "Seid uns zum zweitenmal willkommen" from Mozart's "Magic Flute," in which the boy spirits advise courage and silence.