THE FIRST music in the Kennedy Center was Leonard Bernstein's "Mass" that T opened the Opera House on Sept. 8, 1971. It is hard to think of a more auspicious beginning for the decade of music that has followed.

The Opera House gave Washington its first adequate home for music drama, a beautiful theater which the late Karl Bo hm said had "the finest acoustics of any auditorium in the world except Vienna's Grosse Musikvereinsaal." The Opera Society of Washington, moving from Lisner Auditorium and eventually changing its name to the Washington Opera, profited by the new opportunities in both the Opera House and the Terrace Theater, with operas ranging from Monteverdi to Argento.

The National Symphony grew in splendor and excitement in the Concert Hall, where Antal Dorati moved his new charges into the front rank of American orchestras. He broadened the repertoire admirably before turning over the baton to Mstislav Rostropovich, who continues to spur the musicians to greater heights. One of the particular joys of the hall is its Aeolian-Skinner Organ, a gift of Kay Shouse.

To the Opera House have come some of the world's most famous visitors: the opera companies of Moscow, Vienna, Milan, Berlin and Paris. The Metropolitan Opera, too, now performs there, but thus far rarely with the luster it often achieves in New York.

The newest jewel in the Kennedy Center is the Terrace Theater. Already rich in memories that stretch from Rudolf Serkin's inaugural concert through repeated runs of Dominick Argento's "Postcard from Morocco," the Terrace Theater this season will present five new series of programs.

The briefest samplings of unforgettable musical nights in the Kennedy Center would include Rostropovich and Callaway conducting the Britten War Requiem, John LaMontaine's Wilderness Journal, Dallapiccolo's "Il Prigioniero," and, always, the Bernstein "Mass." Also unforgettable was Roy Harris' final symphony, which drove the audience from the hall.