The Youth Symphony of the United States is a new organization based in Houston, Tex., which plays under conductor Manuel Prestamo. Its 85 players will be heard for the first time in the Kennedy Center next Monday night at 8:30 as the orchestra reaches Washington on its first tour.

Among the musicans, aged 14 to 24, are William Maron, duble bass, from Silver Spring, and Robert Spates, violin, from Bethesda. The program will include Copland's Fanfare for the Common Man, and El Salon Mexico; a dittersdorf Concerto for double bass, with David Allen as soloist; the second symphony by Howard Hansen; and EOS by Collabro. Tickets on sale at the door.

On Thursday, July 26, at 8:30, the National Symphony Orchestra will play at Wolf Trap with Leonard Slatkin as conductor and Barry Tuckwell as soloist in the first of the two horn concertos by Richard Strauss, Slatkin will also conduct Vaughan Williams' Fantasy on a Tallis Theme, and the Bartok Conceryo for Orchestra.

A week from tonight the New York Chamber Soloists will sing Handel's "Acis and Galatea" at 8:15 in the Tawes Theater of Maryland Iniversity. The principal soloists are Jean Hakes, soprano, Charles Bressler, tenor, and raymond Murcell, bass, who will sing with an orchestra of eight. When these same pelple sang "Acis" here a couple of years ago, they made it a great delight.

The Shenadoah Valley Music Festival is now off and running, with concets at 3 p.m., Sundays July 29 and Aug. 5, and on Saturdays, at 8 p.m., July 28 and Aug. 4. Williams Hudson is music director and conductor of the concerts which are played in Orkney Springs, Va.

There will be piping, fiddling, Scottish country dancing and highland dancing in Episcopal High Stadium in Alexandria, starting at 9 a.m., on Saturday, July 28. Among those taking part in the piping compeition will be the Daniel Stewarts and Meliville College Band from Edinburgh.

Prince George's Civic Opera will hold auditions for the coming season on Sunday, July 29, from 7 to 10 p.m., at the Publick Playhouse in Hyattsville. For details and appointment, call 337-1817.