To the uninitiated ear, movie tracks running at high speed through editing machines probably sound like happy hour in bedlam. But to Maysles Films Inc., denizens of a four-cutting-room atelier in Manhattan's cinema district, nowadays it is the soothing burble of a revenue stream. Since being hired to document IBM Corp.'s fiftieth anniversary in 1963 -- one year after Maysles (rhymes with hazels) was founded -- the company has been pumping income into its coffers by producing commercials and movies for corporate customers. At fees up to $400,000 each, industrial films could be a lucrative market for Maysles. But they are simply an adjunct that pays the rent. The real mission of the company is making unflinchingly candid feature-length documentaries -- which, unfortunately, don't.

Clients have found that Maysles Films's industrial work is equally unsparing. Among paid projects are portraits in celluloid of top executives, commissioned so that underlings might get a leisurely look at their leaders in the privacy of a projection room. Maysles cameras have caught IBM's Thomas Watson Jr. hastily extracting a wad of gum, Chrysler's Lee Iacocca spouting expletives like a Nixon tape, and Merrill Lynch's Roger Birk in decidedly un-Street-like badinage.

Such artless slices of life are the hallmarks of "direct cinema," a technique of which Albert and David Maysles, the two brothers who founded the film company, are architects and chief practitioners. Operating as a self-contained film crew, and using portable 16-mm equipment that they developed themselves, the Maysles record the passing scene not so much by observing it but by openly joining it, camera and microphone in hand. In their best-known feature, Gimme Shelter, for example, the direct-cinema lens happens upon a stomach-turning murder at a rock-and-roll concert.

Nothing that dramatic has happened in the halls of commerce, but, for better or worse, the Maysles refuse to employ actors or makeup, and they never stage, retake, rehearse sequences, or even improvise. Says Albert, "We don't know what's coming up next-nor we do we want to." As a result, people come across as "what they are and what they believe in."

Even so, not all subjects are comfortable or convincing in the raw. But, Albert persuades potential customers, "We're in and out very fast. Real life . . . is more convincing." The credibility of the corporation is what customers ultimately sign up for, even if it dabbles with disaster. Only direct cinema could have memortialized the irate New Yorker who came in to yank an account while the Maysles were shooting a corporate paean for Citibank, or the couple who sidestepped on cross-country skis into a bank lobby during a blizzard, or the hungry dog that knocked over a dish of Kal Kan in unmuzzled endorsement. A six-minute Lotus Development Corp. promotion shown at a stockholders' meeting includes a woman executive who can't open a shrink-wrapped company product. "I bench press, too!" she sheepishly admits to the camera. If such probity seems inappropriate to the clients that foot the bill, the Maysles grudgingly grant the option to veto a given episode.

Generally, clients appreciate that unassailable credibility comes at a price. And a good thing, too: The 12-employee company has been taking in a scant $2 million a year. Maysles's first feature, Salesman, a film about a door-to-door Bible peddler, was produced for $170,000 in 1965. But despite its cultist following, in 20 years it is "doubtful it has netted more than $70,000," according to David, who doubles as the company's chief financial officer. A short, Valley Curtain, still is carried as a loss, even though it was nominated for an Academy Award in 1980.

Undaunted, 58-year-old Albert's newest project has him serendipitously hopping long-distance trains in search of real-life stories; David, 53, is working on a nostalgic view of childhood Boston, where he was batboy for the Braves. And to help pay off the ever-demanding muse, Maysles Films is finally contemplating hiring a full-time sales representative.