The Washington Performing Arts Society's concert by the Bryan and Keys Duo at the Kennedy Center Terrace Theater was sold out by yesterday morning. But because of the weather that did not mean a full house. Only about a fourth of the audience made it.

That was too bad, both for the artists and the missing listeners. Not only is this husband-wife team quite proficient, but they gave a special kind of program.

The event was not in the grand bravura style of Rampal or Galway--a Whitman's sampler of Baroque and sentimental morceaux, displaying agility, tone, but not much substance.

Keith Bryan, flute, and Karen Keys, piano, performed what basically was a chamber music program, in which the flute was not always the dominant factor.

All the five pieces were superior and one of them was probably great. That was Aaron Copland's 1971 Duo for Flute and Piano. These melodies were not Copland's most ravishing, but in this case and in most of his chamber music, Copland achieves a sophistication through the sometimes severe cleanness of his lines that few composers have matched.

The jagged intervals of the duo's first movement stood out in stark profile for their expressive daring. From the standpoint of Copland's minimal number of notes some might draw the conclusion that the work is simple; but from any analysis of its intellectual and emotional complications the work is complex and exciting.

There were two other contemporary works much to be admired. One was a relaxed, almost jaunty Sonatine by Henri Dutilleux in which Keys was particularly stylish. It sounded a little like Poulenc.

The other was Bohuslav Martinu's trio for flute, piano and cello, a work that was always savory and sometimes playful.

In the Martinu and the concluding Weber trio they were joined by cellist David Vanderkooi, an ingratiating player.

The management of WPAS is very fond of these performers. Maybe they could give them a break by inviting them back when the odds of the weather are less against them.