Fighting a summer cold and a disorganized Tchaikovsky Competition, a vivacious and talented young soprano from Texas, Laura Claycomb, nonetheless managed to emerge from the vocal section of the competition as co-winner of second prize. Claycomb, 25, has already sung lead roles at leading opera houses, including the one in her present home town of San Francisco. Less than two months ago she appeared at the Geneva opera, where she received high praise for her portrayal of the heroine in Vincenzo Bellini's seldom-heard setting of Shakespeare's "Romeo and Juliet," entitled "I Capuletti e Montecchi."Claycomb's voice, known technically as a coloratura, reaches to the very top of the soprano register. For her second round in the Tchaikovsky Competition, she chose one of the greatest tests of that voice, the long and complicated aria of Zerbinetta from Richard Strauss' "Ariadne auf Naxos." With a fine accompaniment by Australian pianist Phillip Shovk, she battled there only against her cold. An obviously knowledgeable audience, both understanding and overlooking Claycomb's temporary vocal problems, greeted that performance with unqualified approval.For the final round, accompanied by orchestra, Claycomb picked the aria of the Shemakha Queen from Rimsky-Korsakov's "The Golden Cockerel," and the enormously demanding aria of Ophelia from Ambroise Thomas's "Hamlet."Just before her rehearsal Wednesday with Valery Polyansky and the State Symphonic Capella of Russia, Claycomb learned that no orchestral parts for the "Hamlet" aria could be found in Moscow. Frantic calls all over the world ended in failure to unearth the music. Finally, Thursday morning, a set of parts emerged from the archives of the Bolshoi Theater. But they contained cuts which reduced the aria to half its normal length. And time remained only for brief run-throughs with piano and orchestra. That evening, Claycomb took the stage at the Tchaikovsky Concert Hall, in better health but with scant rehearsal to fix in her mind the missing parts of the aria. What followed, however, was a display of technical and interpretive brilliance, including runs into the stratosphere of the human voice, which seemed certain to capture a prize. The voice judges gave Claycomb the second prize, shared with Ukrainian soprano Tatyana Zakharchuk. First prize winners were the Bolshoi's Marina Lapina and a Chinese baritone of considerable accomplishment, Yuan Chen Ye. The grand prize of $10,000 went to Khibla Gerzmava, 24, from the Abkhazian region of Georgia, a soprano with a winning smile and a clear, but still immature voice. She was the audience favorite at Friday's awards ceremony, where she also received the special $5,000 prize. But Claycomb and Yuan would surely have shared a first had the prizes been put to an audience vote.