It's Broadway's dirty little secret. For years, audiences have been largely shielded from a truth of theatrical life that singers and musicians, directors and producers have been privy to: On some occasions, you can't trust your ears.

The three violinists in the orchestra pit? They're being electronically "sweetened" to sound like six. The voices of the rollicking choruses in the Act 2 finale? To give the actors a breather, they're being played for you on tape. The symphonic swell of the overture? It's controlled as much by a guy at a computer as by the person with the baton.

Many Broadway musical moments remain mostly live and somewhat pure, the creation of soaring talents onstage and sawing string players and the like offstage. But more and more, the sound of music on Broadway is being artificially enhanced, with volumes amplified and instruments synthesized as if the theater district were one big recording studio.

"Conductors have lost control of the sound of the orchestra," said Bill Dennison, assistant to the president of Local 802 of the American Federation of Musicians, the union representing Broadway orchestra members and conductors. "It's now in the hands of a technician at a sound board."

The strike by Local 802, one that has been honored by Broadway's actors and stagehands since it started Friday, has brought this trend out in the open. Although producers were threatening to replace musicians at the 18 affected musicals with totally recorded, or "virtual," orchestras if they walked out, the fact is that virtual sound has been creeping onto the Broadway stage for years.

Many on Broadway -- and beyond -- believe that if producers, eager to maximize their investments, had their way, living and breathing musicians would all but disappear. "Broadway is not karaoke," declared Maury Yeston, the composer of the musicals "Titanic" and "Nine," the latter being revived this spring on Broadway with Antonio Banderas. "If we were to follow the producers' plan, we would have to change the lyrics in 'The Music Man' to say '7-point-6 Trombones.' "

The producers maintain that their intention was never to stamp out live music, that they merely wanted to eliminate the contractual requirement for a prescribed minimum number of musicians for each Broadway musical, whether all the musicians are needed or not. For conventional musical comedies in large houses, a Broadway orchestra is typically about 20 to 25 pieces strong; shows with more electronically driven sound, like the rock musicals "Rent" and "Movin' Out," require fewer instruments. In many cases, the union and producers have struck special agreements to allow for slimmed-down orchestras. (One Tony-winning musical of late, "Contact," was danced entirely to recorded pop hits.)

The strike entered its third day today, with no progress reported and no new talks scheduled. (The current offer from the producers is for a minimum guarantee of 15 musicians per show for the largest theaters, down from the current 26; the union wants a base of 24.) While a small number of nonmusical plays were open, picketers once again demonstrated outside many of the musicals' theaters as part of the first labor-related Broadway shutdown in 28 years.

Where musicals are concerned, it is hard to imagine a more central issue than what they sound like. And often these days, on Broadway as well as on the road, critics point out that they don't sound as good as they once did. The piping of amplified voices through a computerized system sometimes has the effect of homogenizing a show's entire sound; sitting some distance from the stage, a theatergoer frequently finds it difficult to locate precisely where a voice is coming from. And because many live orchestras are hidden from audiences -- consigned to sealed pits, or even, as in the case of "Cats," remote rooms -- spectators can forget that real musicians are playing.

"I've heard it so many times," Todd Ellison, conductor of "42nd Street," said today as he picketed outside the Ford Center for the Performing Arts, just off Times Square. "People come up to me at the end of a show and go, 'I didn't know there was a live orchestra under there.' "

Producers say they have no intention of watering down the live experience. "I think audiences do accept a certain trade-off between vocal purity and excitement," said Jed Bernstein, president of the League of American Theaters and Producers, the umbrella trade group for producers in New York and across the country. "But that always has to be kept in balance." It would be foolish, he said, for producers to hurt the quality of the musical sound, because "they'd only be hurting themselves."

The musical actually entered the electronic age decades ago. Scattered microphones were used onstage as far back as the original production of "South Pacific" in the late '40s; it's said that the first body mike was used in the early 1960s to amplify the voice of Anna Maria Alberghetti in "Carnival." (Ethel Merman famously recoiled in horror at the suggestion that she might need an electrical boost.)

Over time, the instruments in the orchestras got their own mikes, and synthesizers were added, too. As computer sound boards came into vogue, to filter all the orchestral and vocal sounds of the show, the connections between the musicians and the actors onstage began to deteriorate. In some pits, the only way the players know the show is proceeding is by staring at closed-circuit televisions.

Dick Latessa, who plays the jokester father of an unlikely teen idol in "Hairspray," says that though the pit is right beneath the stage, he cannot discern any unamplified acoustical sound. "We have the speakers onstage; otherwise we couldn't hear it," he said. "We have some kids in the show who complain they can't hear the music at all. I think they've gone deaf on all their rock music."

Conductors like Ellison acknowledge that it would be difficult to go back to the days when Broadway did not juice up the orchestra, that audiences do not want to have to strain to listen. "People are so used to going to the movies and hearing all the Dolby sound systems," he said. What a show must rely on, he said, is the services of a sound technician with some appreciation of the artistry of each musician in the pit. "42nd Street" was lucky enough, apparently, to have found one. "He keeps it very natural," Ellison added.

Naturalness, though, has become a relative term. Electronic enhancement of sound is such a mainstay now that unions like Actors' Equity routinely make rulings about when it can be used for singers. In "Hairspray," for instance, eight bars of the last number, "You Can't Stop the Beat," are sweetened, said Lon Hoyt, the show's musical director.

"It's uncommon that there isn't sweetening of that kind," explained Dennison of the musicians union. "I don't think any of the big orchestras on Broadway don't have some 'synth' that is replicating, or trying to sweeten" the instruments.

Such manipulations can be a technical aid to a production, "Hairspray's" Hoyt added. "But I think the fight over this becomes when the technology is available to replace real people."

The musicians in the picket lines feel very much as if this strike is about survival. Gordon Titcomb, a guitarist for the forthcoming musical version of the film "Urban Cowboy," said he'd even heard talk that in the event of a strike, the show was thinking of using mannequins in the onstage orchestra.

Whether or not things would ever get that surreal on Broadway, the musicians' strike has drawn a line for the band. How far down the Memorex road do producers want to go?

"Try to think of the magic of the little boys and little girls," said Yeston, the composer. "They always walk down the aisle of the theater and look into the pit. They see 20 musicians down there. What's it going to be like when they see only five? It's like pulling the curtain back on the wizard in 'The Wizard of Oz.' "

Striking Broadway musicians warn of the threat of "virtual orchestras." But actually, most musicals' sound is already electronically "sweetened."