Richard Bales and the National Gallery Orchestra will open the Gallery's 34th American Musix Festival at 7 p.m. Sunday with a program of three Washington premieres: the First Symphony by Robert Washburn; Pioneer Dances by Peggy Coolidge, and the Symphony No. 26, called "Consolation," by Alan Hovhaness.

he festival will continue through May 29, with pianists Clive Lythgoe and Aldo Mancinelli, the Madison String Quartet, a clarinet trio with Lora Ferguson, Elinore Tramontana and Gail McDonald, and a final orchestral program. During the festival special attention will go to Daniel Gregory Mason, Charles Tomlinson Griffes, Edward MacDowell, and, in an all-electronic concert, music by Joseph Ott. All the concerts, as usual, are free.

Antal Dorati is back with the National Symphony this week for his final stint as music director before the takes on the role of principal guest conductor next fall. This week he is conducting all Brahms; in his final week, the Mahler Fifth Symphony.

The New Russian Chamber Orchestra, an aggregation of musicians, recently immigrated from the Soviet Union, is playing at 8 p.m. Saturday at the Jewish Community Center in Rockville and on Sunday at 3:30 a.m. in the Kay Spiritual Life Center, at Amerian University. Tickets will be on sale for the first concert; there will be very limited seatings for the second.

When Ana Maria Vera plays at the Pan American Union at 7:30 p.m. Friday she will be presented with an OAS scholarship for two years' study with Leon Fleisher. This is the first such scholarship to be awarded by the OAS. Ana Maria, who is 12, was born in Bolivia.

At Saturday night's Lincoln Center Chamber Music concert in the Kennedy Center, pianists Charles Wadsworth and Richard Goode will join singers Kathleen Battle. D'Anna Fortunato, john Aler and John Reardon in songs by Schubert and Schumann. Violinist Jamie Laredo and violist Walter Trampler will also play.

Wolf Trap sends word that men interested in being supers in the forth-coming Metropolitan Opera week from June 6-11 should call Gerald Holmes between 9 a.m. and 5 p.m. Monday to Friday, at 938-3810. Pay: $4 per show. (Hmm. That's twice that I used to get).

Cyprus-born pianist Marios Papadopoulos will play at 8 p.m. tonight on the Youth Concerts Foundation program at the Jewish Community Center in Rockville. Tickets at the door.