Murry Sidlin, resident conductor of the National Symphony Orchestra, is leaving that post to take over the job of music director of the New Haven Symphony. While Sidlin's new contract takes effect on Sept. 1, he will guest conduct the New Haven orchestra's four free outdoor summer concerts in June. Sidlin has been with the National Symphony since 1973.

Undergirding the Simthsonian Pupet Theater's current production of "The Magic Flute" is the matchless recording made by Sir Thomas Beecham in the 1930s. Performances are given Wednesdays through Sundays in the West Wing of the Arts and Industries Building, 900 Jefferson Drive. For details on hours and tickets, call 381-5395.

Eric Leinsdorf, who made his debut with the Metropolitan Opera on Jan. 21, 1938, at the age of 25, will make his first appearance with the National Symphony tonight, repeating the program on Wednesday and Thursday.

In theyears since his Met debut, Leinsdorf has been in charge of the Rochester Philharmonic, the Boston Symphony, and, in various assignments, affiliated with most of the world's leading orchestras and opera companies, to say nothing of his long list of recordings in both operatic and symphonic fields.

Out of his vast repertoire, he will, with the National Symphony, conduct excerpts from Wagner's "Meistersinger," and symphonies No. 40 by Mozart and 2 by Brahms. This by the way puts the Brahms Second Symphony on NSO subscription concerts this April 5, 6, and 7, and again next Oct. 18, 19, and 20. Someone is not keeping track!

Last-minute concert announcements: tonight at 8:15 in the University of Maryland's Tawes Recital Hall, Brazilian gutarist Dagoberto Linhares - free. Wednesday night at 8 in the Jewish Community Center in Rockville, on the Young Concerts Foundation Series, hte Chilingerian String Quartet from England playing Hayden, Shostakovich and Beethoven. Tickets at the door.

Robert Shafer's Oratorio Society program on Good Friday night in the Kennedy Center is a distinguished group of French works for chorus and orchestra: thh DUrufle Requiem, Lili Boulanger's "Du fond de l'Abime," and Faure's antique de Jean Racine.Soloists will be Linn Maxwell, mezzo, and Will Parker, baritone.

Pan American Week opens in the Organization of American States Hall of the Americas of concerts on April 11, Marino Rivero, Uruguayan bandonionist; and April 13, Raquel Boldorini, pianist. Recital invitations are available from the Technical Unit on Music, 1735 I St. NW, Room 922.