"Yogurt," intoned baritone Samuel Bonds, "cream cheese pie." Bonds made it sound like he meant it, blending his words easily with the accompanying sounds of piano, trumpet and snare drum.

"Milk that was actually milk." His voice was joined, now, by soprano Marie York Harbison. On the final "milk," they glided down through more than an octave in perfect parallel. A minute later, Harbison was singing instructions to "bake at 350 degrees." This was not a new show being worked kup to compete with Julia Child, but the first rehearsal of "Food" by Pia Gilbert, a witty setting of a text by John Cage which had its world premiere Sunday night in the Capital Hilton Hotel with an audience of 600 women musicians.

The premiere was a highlight of the Sigma Alpha Iota convention here -- the first convention of the international music fraternity to be held in Washington in the past quarter-century. Founded in 1903 and now numbering 65,000 members, SAI claims to be the oldest and largest organization of its kind in the United States. So far it is clinging to its women-only membership rule, except for a few honorary males, although that is causing controversy because of equal-rights legislation. One significant moment of the 78th anniversary convention came on Saturday night when a leadership award was presented to the Evansville, Ind., chapter, which was banished from its campus because of its membership policies.

The music is wall-to-wall at the four-day convention. "We don't have coffee breaks," says chairman Helen May, "we have music breaks." But the music, from avant-garde, through Bach and Schubert to Broadway tunes, is only a part of what is going on. Offstage, there are workshops that range from "Reflections on a Career with a Major Symphony," by violinist Barbara Sorlien and cellist Lloyd Smith of the Philadephia Orchestra, to "Let's Balance the Books!" by national treasurer Elsie Sterrenberg. In small rooms scattered around the hotel's second floor, committees are meeting to work on programs that range from scholarship funds and competitions for composers and performers to the publication of a quarterly magazine, promoting music on television and the transcribing of compositions into Braille for the Library of Congress. The 600 delegates are divided about evenly between students from its 176 chapters on campuses in every state but Alaska and older women from 30 alumni chapters in major American cities.

Musically, the convention spotlight oscillates from students like soprano Wanda Middleton, who gave a brief recital Saturday afternoon, to international stars like soprano Jessye Norman, one of SAI's alumni, who gave a benefit recital last night. Both joined SAI at Howard University, where Norman became the aspiring student and the successful graduate -- a sort of alumni network spanning generations and continents -- is one of SAI's most important elements.

"When you join SAI, you acquire a large, talented family," says Emily Anderson, who was the capter president at the University of Indiana from 1978 to 1980 and is now a member of the chapter at the University of Maryland. "You have the support of all kinds of other people who have been or are in the same boat. You can find a chapter almost anywhere you move, and you are put into contact with people who have been through it already, speak your language and are willing to help you." Her mother, Marilyn, who joined SAI about 30 years ago in Minnesota, agrees and amplifies: "When you're in college, you don't realize what it's going to mean in 10 or 15 years."

Besides alumni, the young SAI member can hope for help from patronesses, prominent members of the local community who are affilated with the fraternity, and from honorary members or "national arts associates," who include such eminent women of music as Beverly Sills, Marta Istomin and Galina Vishnevskaya. Helen May, who brought Vishnevskaya into the fraternity several years ago, still recalls what she said on the occasion: "I'm so happy. I had practically no relatives; now, I have a lot of sisters."

May was also a central figure in SAI's promotion of Norman's career -- a classic example of how the alumni network can help nurture an outstanding talent."I heard her sing," May says, "and I said, 'Jessye, you're going to the top, and we'll start working at it now.' So the SAI's have kind of promoted her, and I got her out to Michigan where she got a full scholarship. pI called up my friends in Michigan and told them to take care of her, and they did. Our patroness chairman at this convention is Sally Fleming, whose husband was president of the University of Michigan."

Perhaps this explains why Jessye Norman, who can draw a capacity crowd to any concert hall in the world, agreed to sing for a few hundred people in the Presidential Ballroom of the Capital Hilton.