John Mauceri will conduct his first National Symphony-Kennedy Center concerts this week. Mauceri, who made his National Symphony debut at Wolf Trap in 1975, conducted the European premiere of the Bernstein Mass in 1973 and the U.S. revival of Wagner's "Rienzi" early this year. He will be joined by pianist Alicia de Larrocha, whose concerto will be the A Major, K. 488, of Mozart.

Mauceri will open the concert will the "Rienzi" Overture, close it with Debussy's La Mer, and include the full version of Charles Ives' Three I'laces in New England, which was premiered by Mauceri in 1974.

The D.C. Youth Chorale will be joined by the Choral Arts Society and members of the National Symphony Orchestra on Thanksgiving Night at 8:30 in the Kennedy Center in a special benefit program for the Chorale. The Chorale's entire program is at stake at the moment, as well as its hopes for a music therapy program for handicapped students.

Thursday night's program will include the Mozart Requirem, Brahms' Song of Destiny, and music by Duke Ellington, Hall Johnson and Roy Ringwald. The special finale will be conducted by D.C. Delegate Walter Fauntroy, who is honorary chairman of the event. Tickets at the Kennedy Center Box Office.

The Friday Morning Music Club's first International Competition for composers was won last year by Christopher Brown of England. His winning second string quartet will be given its premiere on Friday at 8 p.m. by the National Symphony String Quartet, whose members made the final judgment in the contest. Brown, who also won this year's Guinness Prize with his song cycle, "Soliloquy," will be at the performance. Tickets at the door.

Victor Borge, who moves into the Kennedy Center Opera House for the week beginning Nov. 28, holds the record for a one-man show on Broadway: 849 performances. But during his week here, Borge will turn his act into a one-man, one-woman show when he is joined, for the second half of the evening, by soprano Marilyn Mulvey. The show will play nightly through Saturday with a matinee that day. The next day Borge will be back in New York to do a benefit for the families of New York City policemen who are killed in the line of duty.

On Monday, Nove. 28, the Smithsonian's Division of Musical Instruments will put on an 18th-century "Sing In" with Bruce Neely of the University of Illinois in charge of things. Music will be distributed to those attending the free affair, which is to start at 8 p.m. Better get there early.

The National Symphony Orchestra, which spent last Friday recording the Tchaikovsky Violin Concerto with Isaac Stern, under the baton of Mstislav Rostropovich, will spend the day before Thanksgiving recording under the composer's baton, "Songfest," which the orchestra gave as the world premiere here last month.

The composer, who was the first man ever to record in the Concert Hall, is Leonard Bernstein.