The loudspeaker failed. But Peter Greenough was not daunted. "Now you know what it is like being deaf," he said. And then he summed up his speech in a shout: "We are here to help deaf people learn to speak."

Greenough and his wife, opera singer Beverly Sills, had good reason to come down from New York for last night's kickoff of the Centennial Campaign for the Alexander Graham Bell Association.

Two of the five Greenough children are deaf.

Beverly Sills said that when their daughter Muffie graduated from high school, she told her father: "I should tear this diploma in half and give part to you."

"I always knew she could do it," said Greenough, the chairman/manger for the association's foundation. "She went on to graduate from college and now she does book covers for Warner Books. She couldn't come tonight because she is working. The important thing for the deaf is to be part of the mainstream. Teaching the deaf to sign is all right for some, but visual speech, lip reading, makes it possible for the deaf to be part of the world. We are grateful for the help that the Bell Association gave to Muffie."

Other people at the benefit, in tents pitched just outside the Filene Center at Wolf Trap, had equally poignant reasons for raising money to support the education of the deaf.

Former Illinois senator Charles Percy, working his way through well-wishers toward dinner, said, "After the guns of World War II, when I got back to the United States, I thought everyone was mumbling."

Later he told the 450 diners, as he struggled with the faulty microphone, "if you can't hear I could lend you a hearing aid."

Just before the mussel soup was served, Gilbert Grosvenor, chairman of the Centennial Campaign Board, explained to friends that his family has a long history of interest in education for the deaf. "My great grandfather, Alexander Graham Bell the inventor of the telephone was a teacher of the deaf, as was his father. He married his student, Mabel Hubbard. A young woman here tonight has just finished a scholarship established by my grandfather.

"Did you know there are 16 million deaf people in the United States? In this centennial effort we hope by 1990 to raise $16 million to help them."

Before dinner, Lady Bird Johnson, asked if the Reagan administration was dismantling her husband's Great Society programs for the handicapped, said, "Well, there are many programs still working. I know my husband supported the National Technical Institute for the Deaf, still flourishing in Rochester."

Greenough said that the Bell Association, unlike Gallaudet College here and the Rochester school, does not receive federal aid, but Percy said that he was looking into it.

When Percy was in Europe recently he said he was pleased to see "that every time we were in a helicopter it wouldn't take off until everyone put on their ear muffs. We need to put more emphasis on noise pollution. Hearing loss is the greatest disability in the United States."

Grosvenor added that he was "proud of Washington. Of the 15 or 20 corporations we asked for money, not one turned us down." Greenough said the evening raised $145,000.

The association, which Grosvenor said was founded in 1890 by Bell, works to detect hearing loss in infants, promotes the teaching of speech and language, seeks better educational opportunities for hearing-impaired children, provides scholarships, trains teachers and gives information on the causes and treatments of hearing loss.

The guests, who included Ambassador of Japan Nobuo Matsunaga, syndicated columnist Art Buchwald, interior designer Bob Walden and Kay Shouse, the founder of Wolf Trap, dined on herbed loin of lamb and praline souffle' with raspberry sauce at tables centered with Japanese flower arrangements and fans at every woman's place.

The party later moved to the theater for a performance of Puccini's "Madame Butterfly" by the New York City Opera Company. But they still had another half-hour to discuss the dinner. The bus holding half the orchestra got lost on the Dulles Access Road and ended up at the airport.