Two die-hard fans of "A Chorus Line" went to see it twice during its run last week at Wolf Trap. The first time, early in the run, they could hear perhaps a dozen lines of dialogue clearly, and they thought that the sound of the orchestra was distorted. The second time, on the closing night, they report "everything was as clear as a bell." They said it sounded like two different productions.

Classical music fans have had similar experiences; in a half-dozen excursions to the Meadow Center this summer, you could have heard what sounded like a half-dozen separate sound systems. Some of them -- particularly for the American Philharmonic -- were spectacularly good. Others were erratic or confused. At times, as when the cannon began in the "1812" overture, the effect was overwhelming. And at other times, as in "King Roger" on Monday night, it was difficult, most of the time, to tell if the singers were singing in English or in Polish.

What's been happening? The most obvious thing that happened was the fire that destroyed the Filene Center shortly before the season opened, followed by the emergency substitution of a tent that was rushed halfway around the world to shelter this summer's season of performing arts. The tent, for those who haven't seen it, is not really a tent; it is an enormous structure something like a large Quonset hut, a massive framework of metal beams, walled, or perhaps roofed, in hard plastic (nylon net impregnated with polyvinyl chloride). The walls begin above the height of anyone who wants to walk in and curve upward almost in a semicircle until the top where they come together in a peak. A tent would be easier. Canvas is acoustically inert; the Meadow Center structure is acoustically hyperactive.

"You can walk on that stuff," says Alan Perry, the chief of Wolf Trap's four-man crew of sound technicians. "They have workmen up there walking on it, making repairs. You can't cut it. You can't puncture it; it acts acoustically as a hard, reflecting surface. Go in there and clap your hands, and you can hear reverberation. There is an acoustical field."

This field is a particularly problematic one, according to Wolf Trap's sound designer, Farrell Becker. "When they first showed me the photos of that thing, with its curved side walls, I almost dropped dead," he says. "Curved surfaces tend to focus sounds and make them pop up in odd places. If you're sitting in a certain part of the audience, you may hear a harp 13 feet over your head. In another place, you will hear a lot of violas."

Since Wolf Trap opened to the public in 1971, its sound system has been operated on a consistent philosophy -- actually a split-level philosophy. Pop acts usually come in with a clear idea of the specific sound image they want to generate, and Wolf Trap technicians are there to help them achieve it, on their own or with the technicians who travel with the act. For classical music, the goal is to sound natural -- to create the feeling that no amplification at all is being used. In fact, inside the Filene Center none was used, and it is used sparingly inside the Meadow Center.

The problems do not arise with the complex, state-of-the-art electronic equipment -- $96,000 worth of it, on loan from 11 manufacturers, including such respected names as JBL, Electrovoice, Crown and Interface Electronics. "With this equipment, you can make Barbra Streisand sound like Dean Martin," says Perry proudly. What causes trouble is the natural part of the sound -- the voices and instruments bouncing erratically off those hard, curved walls.

One of the things that made "A Chorus Line" sound better late in its run was that the man on the mixer had been working in the Meadow Center for a few days and had become familiar with its peculiarities. But the improvement really became spectacular when the orchestra was kicked out of the theater. "I wanted that orchestra in a trailer a block away," recalls Perry, "and when the show's sound designer came down and heard it, he agreed. What we finally did was put the orchestra on a loading dock with three layers of velours around it to muffle the live sound, and then pipe the sound in."

One night early in the season, the lawn was almost totally empty; a torrential rain had come in to welcome the American Philharmonic to town. But for the few hardy souls huddled under plastic sheets, the sound there came through as clear as a bell -- rich, vivid, beautifully balanced and finely detailed. That experience, confirmed at later concerts, was enough to make one wonder whether the cheap seats aren't the best bargain this summer at Wolf Trap.

Loyal employes will not comment on this idea; they would like to say that every seat in the house is a great seat, but they can't quite bring themselves to do that, either. The fact is that the Meadow Center, marvel of technology that it is, remains a makeshift expedient which will have to suffice until the Filene Center is replaced. Until then, the walls will continue to bounce back high and midrange frequencies in erratic patterns while letting the bass frequencies leak through. In some parts of the tent, a glockenspiel may leap out of the orchestral texture, while elsewhere it may be a solo flute. Once you understand what is happening, it can actually be an interesting experience; and since the seating is unreserved, there are many chances to experiment.