IT'S TIME to take up the rug. Where else in the world is there a concert auditorium, chamber-music recital hall or opera house in which, every inch of the floor is covered with carpet -- except in the Kennedy Center's lovely Terrace Theater?

When the Terrace opened there was a lot of surprised murmuring about the deep purple ruggery that spreads over not only the aisles but under every seat, covering the tiniest parts of the floor. As a result, the sound of every musical offering has suffered from the deadening effect of the unwelcome purple layer. There is no question of clarity: the acoustics of the hall are bright and clear, letting you hear every nuance in the performance. But the sound lacks any reverberation, any feeling of vitality in the tone.

This has been as true of Rudolf Serkin's poetic pianism, which opened the Terrace, as of the concerts by the Theater Chamber Players, whose new home the theater is. And it is becoming an increasing source of concern to the performers. Phyllis Bryn-Julson, one of this country's great singers, who has often sung in the hall, says, "I don't get anything back from the theater. I am used to hearing some sound returning."

Dina Koston, the director of the Players, has tried various experimental arrangements of the musicans on stage, but the carpet defeats all attempts at increasing a sense of vibrancy in the sound. Paul Hill, who is presenting his new Washingtons Singers in the Terrace Theater four times this season, is deeply concerned that the voices will seem to lack their natural ring.

Susan Wadsworth, the head of the Young Artists Series, which is celebrating its 20th anniversary in New York this season and is presenting five outstanding younger musicians in the Terrace Theater is similarly worried. She fears that a young cellist on her series, who has a splendid tone, may be heard without the rightful overtones. Already this season two pianists, a chamber ensemble and a singer have given concerts there and have suffered from the lack of resonance.

The matter is of special importance at this time because of the marked increase in bookings of purely musical attractions into the Terrace. The Washington Opera is offering 31 performances there; the Theater Chamber Players 10. The Washington Performing Arts Society, moving a substantial number of its presentations to the Terrace, is bringing in 18 dance evenings, six piano recitals, two song programs and a cellist. The Young Artists Series numbers five events and the Paul Hill Chorale will be there four times. Furthermore, as the Kennedy Center moves toward its 10th anniversary season next fall, it is planning additional music and dance events in the terrace as a part of that anniversary, while the Center's new artistic director, Marta Istomin, is making major plans to increase the amount of chamber music.

For purposes of comparison, Washington has one of the greatest halls in the world for the very kinds of programs that are offered in the Terrace Theater: the Coolidge Auditorium in the Library of Congress. Anyone who wants to know how a Serkin or a Bryn-Julson, a chamber ensemble or an opera can sound in a theater which is very close to the Terrace in design, layout and dimensions, can find out -- or has already -- by hearing the same artists at the Library.

When the Terrace Theater was new, its superb acoustician, Cyril Harris -- who was responsible for the excellent acoustics in the Concert Hall, the Opera House and the Eisenhower -- sa id that he felt they had achieved the "best possible results in the Terrace, in view of the compromise required to provide a theater for concerts, opera and intimate theater."

Yet examples in all three of these areas will suffice to indicate that the Terrace should not and need not lack the kind of live sound heard at the Library. Some years ago, one of the great events in the history of the Coolidge Auditorium took place when the late Sir Thomas Beecham gave a memorable lecture on the subject of Mozart. Speaking in his inimitable manner, with a voice of modest proportions, and often speaking as he illustrated at the piano, Beecham was heard easily throughout the hall.

The world premiere of Gian Carlo Menotti's madrigal fable opera, "The Unicorn, the Gorgon and the Manticore," was given at the Library of Congress under Paul Callaway's baton, and there was no difficulty whatsoever in hearing and understanding all of Menotti's somewhat exotic text. And no more intimate theater could be imagined than that provided last spring by Sir Peter Pears who, for the entire second half of his program in the Coolidge Auditorium, spoke and sang in a quite, beautifully modulated voice: that was heard effortlessly throughout the hall.

The Sole vital difference between the Coolidge and the Terrace is that at the Library there is no absorbent rug. Without its rugging, the Terrace Theater could rival the Coolidge Auditorium without sacrificing a whit of its desirabliity as a legitimate theater -- and perhaps enhancing the spoken drama. Opera tends to sound rather well in the Terrace, thanks to a well-designed orchestra pit that can be raised or lowered, and to the projection afforded the voices by the presence of sets behind them. But opera, too, would benefit noticeably by greater reverberation.

And speaking of benefits: Concerts are not meant to depress those who attend them. Yet three concerts in a row in the Terrace have presented the artists properly placed well to the front of the stage. But behind them? Total darkness. So dark that you cannot even see the black curtains at the back of the stage. It is like looking into the Back Hole of Calcutta to sit there and hear these attractive musicians trying to overcome the handicap of a dead hall in front of the darkling gate to some shadowy inferno.

There is no reason why the Terrace Theater, one of the most attractive theaters in the country, should not look and sound as beautiful as its imaginative combining of pink, silver and purple suggest. The stage lighting is up to those who engage the hall. The quality of sound is something that they and the Kennedy Center management should improve without delay. Take up the rug!