A glorious choral sound filled every corner of the Kennedy Center last night. Over 300 singers from the Oratorio Society and the Choral Arts Society stood, rank on rank, on the new structure built to hold the largest chorus ever to sing with the National Symphony in the Concert Hall. The stands will be used again next April when Erich Leinsdorf conducts the Mahler Eighth Symphony with even larger forces.

Last night's music was the Berlioz setting of the "Te Deum." In addition to the onstage singers, 40 more children were divided into two choirs in the front boxes. If that sounds like a lot of voices, Berlioz had 900 when he led the first performance of the work in the Church of St. Eustache in Paris in 1855.

He said its closing pages surpassed "all the enormities I have ever been guilty of before." Enormous or not, the work is uneven in inspiration. Between impressive sections, the second and third episodes fall into routine platitudes.

However, there are glories enough scattered through the work: orchestral writing of arresting beauty, choral moments that overwhelm by their sheer strength. Both of these were handsomely delivered last night as Mstislav Rostropovich held the work together, making the most of its strong passages. The choruses, adults and children alike, were marvelous.

Berlioz deivsed organ sequences, both solo and in dialoque with the orchestra, that range from the instrument's full power to its quietest stops. The Filene organ filled these spaces impressively. The single solo in the "Te ergo quaesumus" was sung by British tenor David Rendall. It must be said that it was notably better served last spring when the music was heard in the Shrine at Catholic University. Rendall's singing was rather ineffectual.

Rostropovich preceded the Berlioz, which was a first for the National Symphony, with what proved a truly gala reading of Samuel Barber's Second Essay. Perhaps the placement of the orchestra, on the temporary stage that carried them out over the first row of seats in the auditorium, gave them greater luster. Choir by choir in Barber's eloquent writing sounded superb, especially horns, timpani and the strings.