Richard Lert, at 91, will conduct the Brahms-Hadyn Variations at the Shenandoah Valley Music Festival in Orkney Springs, Va., at the 3 p.m. concert on Sunday. The following week, on July 31, Lert will conduct the Beethoven Third Symphony. Other works on those programs will be conducted by fellows attending the 1977 Conductors Workshop under Lert's direction.

Tickets are available from Festival, Box 12, Woodstock, Va., 22664, or by calling 703-459-3396.

Out-of-town festivals include those now in progress at Malboro, Vt., where Rudolf Serkin and his friends and colleagues make music at 8:30 p.m., Saturdays and 3 p.m., Sundays. There will also be Friday concerts on July 29, Aug. 5, and 12.

Since space at Malboro is always tight, you might want to check in advance, bu calling 802254-8163, or writing Box R, Malboro, Vt., 05334.

The National Symphony concert on Saturday, will be under the direction of Everett Lee, with George Shirley and Joy Simpson as soloists in music by Bizet, DePaur and Tchaikovsky. Meanwhile, the Rudolf, Max and Firkusny, continue their Beethoven concerto cycle on Friday and Sunday nights, surrounding Friday's Beethoven with Mozart, Britten and Wagner.

The Baltimore Symphony, playing at the Post Pavilion in Columbia, on Friday and July 29, under the baton of Andrew Schenck, will offer Rodgers and Hammerstein: Friday with soloists and chorus and July 29 with duo-pianists Ferrante and Tiecher.

Radio Wagner: A live broadcast from the historic stage at Bayreuth, Germany, on Sunday will bring listeners a complete performance of "Tristan and Isolde" between 10:45 a.m. and 5 p.m.

Those times are correct: Bayreuth gives its opera uncut with long intermissions.

The cast will include Catarina Ligendza as Isolde, Yvonne Minto as Brangaene, Karl Ridderbusch as King Marke. Donald McIntyre as Kurvenal, and a new tenor, Spas Wenkolf, as Tristan. The conductor will be Horst Stein. The broadcast will be heard locally over station WETA.

The Washington Civic Opera Association will present a concert version of Bizet's "Carmen" on Thursday, July 28, at 8:30 p.m., in Constitution Hall. The program will be free and will feature Sandra Walker in the title role, John Sandor as Don Jose, Carl Gerbrandt as Escamillo and Jane Stephens as Micaela. Richard Weilenmann will conduct.

The Fifth Annual Musical Showcase Competition will be held in Miami on Oct. 14, 15 and 16, with a first prize of $4,000 and second prize of $1,500. Pianists between 17 and 28 on Aug. 15 are eligible. For details write Joseph P. Naughton, Competition Director, Financial Federal Savings and Loan Assn., 8991 S.W. 107 Ave., Miami, Fla., 33176.

Internatinal Congresses Of Organists are held only once on 10 years. The first was in London in 1957, the second in Canada in 1967. This year the Congress, coming to the United States for the first time, will met in Philadelphia on Aug. 1-4 and in Washington on Aug. 5 and 6. The final sessions will be a festival service in Washington Cathedral at 5 p.m., on Aug. 6. with organist Paul Callaway, with brass, and the Cathedral Choral Society.The public isinvited.