At 11:50 yesterday morning, soprano Teresa Radomski stepped on the American University music department stage for her moments of scrutiny in the Rockefeller competition. It was an audition, 15 minutes that could change her career.

What she faced was a phalanx of five judges, arrayed in a cluster about 10 rows back in the semidark hall, sitting behind music stands with little lights. The stands were loaded with scores of a remarkable array of music--most of it obscure.

They might have been five interrogators giving the accused the third degree.

"May we hear 'Eloquence' by Griffes?" panel chairman Thomas Paul firmly inquired.

And Radomski, accompanied by her pianist, sang this short, tender and all too seldom heard gem of an elegy by the early 20th-century American Charles Tomlinson Griffes.

Then the tension broke slightly as one of the judges threw her a compliment, which was seldom the case during the full days of testings Sunday and yesterday: "Thank you. That was lovely."

Then Paul requested, "Would you do one of the Schwantner songs for us?"

"Yes, I'll do the second one," she replied.

This daring descant by contemporary composer Joseph Schwantner sounded treacherous both in pitch and rhythm, and it sounded like Radomski did very well.

She finished the song and was dismissed with a polite, "Thank you." Singers are scored on a scale of 1 to 100. Radomski will be notified by mail.

The contest is called the International American Music Competition. Among musicians it's called the Rockefeller competition. And winning first place can, simply, make a musician's career. In effect it means $50,000, which the organizers say is the biggest prize in music. And then there is the acclaim.

About 270 singers around the world will compete for it this year. Twenty-seven singers competed in the preliminaries at American University; the ones in San Francisco and Chicago are past, and those in New York and London are yet to come.

Here, in the music building's recital hall, they came in quick succession. "It's lonely up there," remarked Nancy Hill, a young soprano from Austin, Tex.

But the judges were trying to be nice, too. They are singers too, and one of them, William Parker, a former Washingtonian, was the first-prize winner last time.

Yesterday Parker, now a noted baritone, described what it is like: "The biggest pressure is that you have to excel at all the different functions, because you don't know what any particular judge is most interested in. All at the same time you have to concentrate on high notes, line, intonation and all the varieties of style. And of course you have to communicate."

The Rockefeller competition is not just another music contest. It was set up in 1978 by Howard Klein of the Rockefeller Foundation. The idea was to spend a lot of money to rectify two glaring inadequacies in the music world: neglect of American music and neglect of the recital repertoire. One year it is for pianists, one year for violinists and the next for singers. Pianist Bradford Gowen, now a professor at the University of Maryland, was the first winner.

The winner gets $10,000 in cash, plus another $5,000 for demonstrating "continued commitment to American music," plus $35,000 for "career promotion," a Carnegie Hall recital and a recording contract.

Another unusual feature of the Rockefeller is that there is no age limit. Parker, for instance, won when he was 36. "They asked me to be a judge that year," he cracked, "and I entered instead."

Parker, who appeared as the delightful Papageno last fall in the Washington Opera's "Magic Flute," explained the contest's appeal for him, in addition to the money: "I am a lyric baritone and we aren't very important in most opera stories. The major opera companies and conductors don't break their backs to get us the way they do the tenors and the basses; we don't make or break that many operas. And as recital artists, there wasn't that much interest in us, particularly in Europe, because they think the art song is theirs and they don't need to hear us.

"So I found that my career was falling between the cracks. It was not as far along as it could or should have been. Menawhile, I loved American music, so I shot for this."

The guidelines for deciding what the contestants are to perform are quite complex and involve a rigorous grounding in American music. The object, as Ellen Buchwalter of the Rockefeller Foundation explained, is in fact "a marketing device": If you want the prize enough, you will learn the music and, win it or not, you will then know it and sing it.

Washington soprano Linda Allison said yesterday that most of the music will go into her repertoire. But she admitted that David del Tredici's "Four Songs on Poems of James Joyce," one of which she sang at her audition, "may come a little slowly. It's incredibly difficult."

No winners are chosen until all preliminary auditions are over. Then the field will be narrowed to "somewhere between eight to twelve," according to Willa Rouder, who coordinates the contest for Carnegie Hall, the cosponsor.

Semifinals and finals will be in New York in September. Among the judges then: Phyllis Bryn-Julson, Sir Peter Pears and Elizabeth Schwarzkopf. As grand inquisitors go, a trio to be reckoned with.