At night, if you stand among the trees and listen, music wells out of the dark sky all around you, like dew.

From a cabin hidden in the maples a piano ripples through Tchaikovsky. From the study hall wafts a Mendelssohn sextet. From the open dorm windows, a violin, a cello, a dazzling fragment of a Bach string duet. Faintly, across the valley, a ghostly flute trills. 'A Temple of Music'

Pablo Casals dropped by here for a short visit in 1960, stayed for two months to play the cello and conduct, and came back every summer for the rest of his life.

He called it a temple of music. Someone else said it was a musical powerhouse. Its actual name is the Marlboro Festival of Music, but it is more than a festival, more than a music camp, more than a summer school.

Pianist Rudolf Serkin, the guiding genius, once said Marlboro's concert audiences are valued "for acoustical purposes only." In the early years, so few people found their way here that the folding chairs were spaced far apart to give the illusion of a full house. But now the season is sold out by April, three months before it even starts.

Musicians come here to play, not to perform. They come as young as 18 --in special cases, 16 -- but none of them is what you would call a student.

"They aren't even considered unless they have complete technical mastery of their instruments," said administrator Frank Salomon. "That's the starting point."

As a visiting artist observed, "Everybody can play everything." Rehearsals

An intense, dark young man, barefoot and wearing a "No Swet" T-shirt, pulls up his chair to join the circle at center stage. While a girl pulls delicately at tiny overhead mikes, he tunes his violin, and the other musicians follow his sweetly drawn A.

"I want to take it from R, here," says Peter Zazofsky, who as first violin seems to be the leader. Michael Tree, violist of the Guarneri Quartet and the senior member of this group, hasn't arrived yet. They are playing Tchaikovsky's Sextet in D Minor, and the concert is tonight. They have been rehearsing it for three weeks, and there is still work to do.

Thirty people lounge in the hall, an airy barn of knotty pine with great laminated curving ribs that make you feel you are sitting inside a giant cello. One woman stolidly reads a newspaper. Glass doors at the side open to grass and maple trees and, beyond the road, the hills of Vermont.

"Judy, are you ready?" Zazofsky shouts to the sound engineer, now off-stage. They will be taping later. Michael Tree strides in, and with hardly a word they are off.

Jerry Grossman is the lead cello; the second violin, viola and cello are played by younger men, the ones you are not supposed to call students.

Something is wrong in the second movement. Zazofsky wants a passage played again, again. They take it apart, at half-speed. Six bars. One bar. One note. Dismantling it. Pieces strewn all over the stage. Then they put it back together and go on, and the music builds until all six bodies weave and sway in unison like seaweed in a tide, swept up by the passion, utterly absorbed. The woman is still reading her paper. The Beginning

Marlboro really started in 1950, when the great chamber music artist Adolf Busch, his son-in-law Serkin and some others summering in these pleasant hills between Brattleboro and Bennington decided to give a few little concerts in the vacated hall of newly founded Marlboro College.

By the next year Busch was dead, but Serkin and his friends carried on.

"We consistently have to resist the pressure to expand," Salomon said. "We have about 70 musicians, some very young, some more seasoned professionals and a few older ones, masters like Mieczyslaw Horszowski, who's 85, and Pina Carmirelli, Felix Galimir, Mischa Schneider and Marcel Moyse, at 83. This year we have two families of musicians from Italy.I'd say with families and staff there are 140 people here.Most of them live in the dorms, some rent cottages in the area."

Younger participants are asked to contribute $1,500 each if they can. The senior people get a rent allowance. The festival -- which this year ran from July 8 to Aug. 13 -- is kept afloat by grants and gifts, and there are the record sales and the concert tickets ($9 top). (The 1979 season, expected to start on the weekend after July 4, will run for six weeks and feature 15 public concerts. Advance information is provided by the winter office, 135 South 18th St., Philadelphia, Pa., 19103).

But nobody gets rich at Marlboro. In fact, it is the other way around.

"Remember, every one of these people is a professional," Salomon said. "Every one of them is giving up from $3,000 to $50,000 in concert money to come here."

Participants already established are admitted by recommendation; the newer talents get auditions. Each person is asked to list the works he or she would like to study, and then Serkin and other key musicians make up a complicated schedule.

"They have to match up the people with each other, and the people with the music, and get at least one senior into each ensemble. A person may ask to play Bartok's First Quartet but wind up with the Third. We don't do many quartets because they take so long to master, but we might try a rare or especially interesting one. In one season we'll do 225 works by 84 different composers. Maybe 80 works a week."

A musician will get 12 to 28 hours of rehearsal a week. Most works are rehearsed for two or three weeks, but only a handful are performed. Some, like a particularly rewarding Schoenberg quartet, may be rehearsed for the entire two months and still not be performed.

"Nobody knows until a few days before the concert what will be played," Salomon said. "When you buy your tickets you don't know what you're going to hear." It's Not All Work

In the dining hall everyone lines up for lunch. All hands take turns working on the crew, a week apiece, so your waiter is apt to be a distinguished cellist, and your busboy could well be Rudolf Serkin.

Across the table, baritone David Evitts massages his throat. The wet summer has been hard on vocal cords, he says.

Someone throws a balled napkin at another table. He is promptly beaned in retaliation. Soon napkins are flying everywhere. Serkin is one of the worst offenders, eyes flashing behind thick glasses as he hurls napkins with gentle glee.

Announcement time. A staffer raps a glass for attention. Somebody picks up the rththm, and within moments you can't hear yourself think for the roar of cups, silverware, plates and empty chairs being banged in allegro vivace time.

"Thank you, thank you," shouts the staffer, bowing. "Everybody who has dishes in their room please bring them back. This means you!"

It is the last day of rehearsals. Outside the dining hall, people sit on the granite hummocks that poke up from the sparse New England grass. Serkin roams among them in his suspenders. There are a lot of couples. Two young violinists with their cases sit on the steps talking very privately. At least 50 marriages started at Marlboro. Stars Every Year

"Stars? Every year we have stars," someone said. This summer everyone is talking about the fierce talent of pianist Cecile Licad: It is her second year here, and she is still only 17 and wears braces. She comes from the Philippines but has lived with her mother in Philadelphia for the past six years.

Now she is on her own for the first time. This winter she will live at Serkin's farm in Guilford, Vt., along with a few other very special young people.

The talent comes from all over the world. It also comes from just down the road. Cellist Eric Bartlett was raised in Marlboro. His mother is a teacher in the local schools and his father is a policeman besides being a sound engineer. He directs traffic for the concerts.

"The goal," muses Serkin, intercepted on his way to yet another rehearsal from yet another meeting with a grants committee, "is to find the most gifted young musicians and hope they learn the difference between technique and art. We hope they will learn insights. Everyone learns, and everyone teaches."

Often a member of a famous quartet spends a summer here to see the music from an unfamiliar aspect. Often the most distinguished and famous play second desk.

"There are many approaches to music," Serkin says. "Ours is one of commitment and intensity. There is a feeling of joy."

In Europe they speak of Marlboro Music, meaning this quiet consultation, this mutual search for the truth in music, this Quaker-like sense of the meeting. Glimpses

A girl violinist trips deftly through a tortuous Stravinsky passage, her blond hair flying up at each staccato note. She pulls a droll clown face to match the music. . . . Peter Zazofsky, deep into the Tchaikovsky, flashes a brilliant smile at the second violin, Ralph Evans, when they hit a difficult attack just right. . . . Cecile Licad, tense as a cat, hands poised at the keys, waits for her next entrance. . . . Serkin, at the piano in concert, listens as the three women players, one of them his daughter Judith, set Brahms afire. They have been timid in his presence, but now they are opening up, letting go. There are 26 pianos on the campus. Serkin Is Marlboro

It is not always idyllic, of course. With all that sensitive talent living at close quarter [WORDS ILLEGIBLE] resentments over parts assigned, performances scheduled, attention paid. If you are giving up $20,000 in concert dates to come here, and then find that someone else will be doing the Smetana work you had counted on, you may be a bit upset.

There is also jockeying for places in the "Music From Marlboro" touring program that gives chamber music concerts in some 50 cities through the winter. Now and then one hears complaints that the festival is somewhat family-ridden, that it helps to know the right people, have the right relatives. But this doubtless would be said of any enterprise which had a dozen applicants for every berth.

The important thing is: Marlboro works. Three celebrated string quartets -- the Guarneri, the Cleveland and the Vermeer -- were formed here, as were the Orpheus Trio and the Aulos Wind Quintet. And Serkin himself seems to be a healing spirit, popping up everywhere, touching people, talking, listening.

In a way, Serkin is what Marlboro is all about. At 75, the famous pianist, divides his time between the Curtis Institute in Philadelphia and this feast of music which has taken on some of his radiant personality.

He wears his fame rather absently, like a half-forgotten but ever-present amulet. It can still get in the way, though there is an austerity about him that keeps all but the most brash strangers from rushing up.

"I was at a concert at Bennington," he recalled, "and they played a Varese. I didn't like it, it was terrible, and no one clapped. So I thought, somebody should clap a little, at least, so I clapped. So they played it again."

His fingernails are split and chipped from a lifetime of banging on ivory. Finale Time

It is the last Sunday afternoon, time for the traditional finale, the Beethoven Choral Fantasy in C Minor, a neglected work that sounds like a study for the Ninth Symphony.

The place is packed. The audience, mostly middle-aged, seems musically sophisticated. Some have been staying at motels as far away as Wilmington, 10 miles down the Molly Stark Trail. A few have been here all summer, gorging on music. Contributors get passes to the final rehearsals, but anyone can wander about the campus any day of the week, stretch out on the grass beneath almost any window, and hear the raw stuff. (In the restroom a sign: "Please do not flush toilets when concert is in progress.")

The orchestra is crammed onto the stage, and behind them, jammed in three deep, stands the chorus. An amazing group.

There is Cecile Licad. There is a woman from the business office. There is a neighboring farmer, sunburnt face above formal collar and tie. You recognize people you have seen around the place: kids, very old ladies, bearded men with canes. Among the soloists you spot David Evitts. And in the orchestra you pick out all your friends, faces you know by now, the blond girl violinist, Peter, Eric, Jerry, Michael Tree.

Now Serkin, at the piano, plunges into the music. The chorus watches, fascinated. Then draws a united breath.

This is Marlboro Music: something that goes beyond dusty precision, transmuting sounds into feelings. In a few hours everyone will be going home, the instruments will be packed, the laden cars will be pulling out, disappearing down the road, scattering. But for now everyone is here, together, evoking the rage and joy of Beethoven, and the moment is all there is.