Trumpet blasts from the mariachi band roared through the Kennedy Center atrium, where National Endowment for the Arts staffers were drinking from fat wine glasses full of pink, grapefruity concoctions.

While most staff members chatted away, one was busy at his other job, playing the trumpet with Mariachi de Las Americas.

"I've got 17 different cards," said Dan Sheehy, PhD, ethnomusicologist and trumpet player with Mariachi de Las Americas, between trumpet performances. He pulled two out of a jacket pocket. "I feel like Rockford."

Sheehy, a folk art specialist with the NEA, was much happier playing than standing around schmoozing -- even though this was basically a party for him and all the other staffers who run the NEA.

The occasion was a celebration of NEA's 15th anniversary yesterday, and served as a reason to recall notable moments in the history of the federal grant-making organization.

"Every five years I get a chance to thank the people who keep the Endowment going," said Roger Stevens, head of the Kennedy Center, who was the first chairman of NEA. "It seems to be a lot of people now. Remember how many people it was when you and I did it, Liv? Fourteen or 15?"

Livingston Biddle, NEA chairman since 1977, spoke up: "Four, to be exact."

"Well, you've been doing well," said Stevens to Biddle as guests listened.

"You've managed to avoid the bad publicity I get . . . If this nation can spend $100 billion on education, they can spend one percent on arts. So that's your next job, Liv -- to get funding up to $1 billion."

Actually, a Senate appropriations subcommittee recently slashed $4 million off the NEA's request for $160 million for the coming 1981 fiscal year. Last fiscal year the NEA received $154.4 million.

"It's not the easiest season," said Don Moore, a deputy chairman of the NEA. "Appropriations are up in the air. We just finished one fiscal year . . . "

Irvine MacManus, assistant director of NEA's museum program, shook Moore's hand. "Happy Birthday, sir," MacManus said.

"Thanks. I don't look a day over 14, do I?" said Moore. (He later repeated this in his remarks to guests, earning groans all around.) "This guy was a chauffeur for Nancy Hanks [a former NEA chairman]," said one woman, pulling Robin Murphy by the arm. "We call them clerk/drivers -- that's Civil Service terminology."

That was 5 1/2 years ago, when Murphy was a year out of Kenyan College and out of work. A housemate told him the NEA was looking for people to do odd jobs. He landed clerk/driver. Now, he's the administrator -- the third-ranking person -- in the theater program at the Endowment.

For the occasion, Livingston Biddle broke with his usually calm and benign demeanor to do something as unorthodox as sing a song.

"With apoloties to any musician here," said Biddle, a T-shirt proclaiming "You've gotta Have Art!" pulled on over his shirt and tie. He began a song he had devised to the tune of "What a Difference a Day Makes:"

What a difference 15 years make ;

24,000 applications . . .

We're 15 years old today

And I'm here to definitively say

Our achievements are here to stay

Just because we have you ."

Also wandering through, flute in case, was Ezra Laderman, head of the NEA's music program. "I was going to play," he said, looking at the mariachi band, "but they're having too much fun."

This evening, the NEA and the National Endowments for the Humanities -- also celebrating a 15th anniversay -- will be jointly feted at a White House reception given by Mrs. Carter. "This is just the warm-up," Don Moore grinned.