JUST WHEN you thought it was safe to go back in the elevator, Muzak is changing its tune.

As of Jan. 1, the sound-conditioning system that cools 80 million Americans a day (and heats up some with esthetic indignation) will introduce an option: The company with a 30-year reputation as purveyors of innocuous instrumental background music will offer louder, vocal pop foreground music. America, are you ready for "Tones?"

Muzak -- whose 3,000 clients in the Washington area alone include Geico, McDonalds, the Pentago and CIA (which uses it to obsure conversations) as well as shopping malls, restaurants, boutiques and thousands of offices -- believes the answer is yes.

Tones is "a big radio station," says Rod Baulm, vice-president and director of programming for Muzak. Whereas Muzak records its own instrumental versions of well-known songs, Tones will simply lease existing masters of current hit records -- vocals and all -- and arrange them on tapes: without commercials and without song or artist identification. Tones will be aimed at upbeat locations like boutiques, restaurants, singles bars and other "lifestyle centers."

"Our franchises have certain locations where Muzak just doesn't satisfy the needs of the clients," Baulm says. "They want pop music and they want the real thing."

And many of them have been getting just that for free by hooking up FM radios to sophisticated speaker systems, a practice that not only offends opponents of forced hearing, but has drawn the eye of the Supreme Court and the wrath of Muzak. In a 1975 decision in which Muzak was heavily involved, the Court ruled that radio could not be played in a public place over loud speakers for "seeming profit" without that establishment paying a licensing fee to ASCAP/BMI. ASCAP is currently in the New York District Court with what they hope will become a landmark case against The Gap Stores, charging copyright infringement over The Gap's practice of using radio programs without paying authors and publishers for their services.

"If you can get away with piping it in for free, why pay $800 a month" for Muzak, asks Bob Chanler of WGAY. (The actual fees are considerably lower and graduated according to the number of subscribers, says Muzak, which had sales of close to $200 million last year.)

Muzak's major problem at this point is that some of the biggest record companies (CBS, Polygram, RCA and Capitol) are refusing to cooperate in leasing the material: No pay, no play. Muzak wants it for free, claiming that Tones will provide the same kind of promotion that radio stations give records. "CBS won't even talk to us," says Baulm. "They'll cooperate when they find out what we can do for them. I'll be even more persuasive when we break an act for them. We can help expose acts and sell records."

"We don't believe that," says Arnie Holland, director of business affairs for Capitol Records. "There may be a slight promotional benefit, mostly subliminal, but no one will know who is singing. Our feeling is: 'If you want to use our music, pay for it.' They're taking something we own, making a lot of money with it, and we want a share for ourselves and our artists."

(The only "foreground" service that has come up with a payment scheme is Oz Productions out of Colorado, which pays on per-cut, pro-rata basis against the gross. "The idea is hitting home," says Holland. "We think they'll come around.")

Album-oriented and middle-of-the-road FM stations are not entirely pleased with Muzak's move. These stations, heard in hundreds of retail locations in the Washington area alone, receive no fees for their use but profit in reputation and exposure. Tone and FM will exist in about the same relationship "as Muzak and the so-called Good Music listening stations have now," says Hal Smith, owner of the Washington Muzak franchise. "People who want music for productivity, efficiency and morale cannot get that from radio. Radio is music that picks you up one moment and oftentimes knocks you down the next."

A listing of artists on a sample Tones tape -- Doobie Brothers, Tim Weisberg, Fleetwood Mac, Steely Dan, Commodores, Diana Ross -- indicates a safe middle ground not much different from what one would hear on any "Platinum Plus (conservative superstar music) stations, but without the commercials.

"Tones is not unlike what you hear on the radio," admits Smith, claiming nonspecifically that "it's how we program it that's going to be different."

Tones will certainly not be a "tool of contemporary management. It's for a specialized audience, for lifestyle shops. It preempts the Muzak principal," says Baulm.Adds Smith, "for people working under repetitive work tasks, Tones would be unnerving, attention-getting, an entertaining value rather than one of productivity and morale. It's the commercial establishments with the young people's themes that are very interested."

Apparently enough of them are interested to create a potentially large new market -- and many small, competitive "foreground" music firms have sprung up in the last two years. "Rather than let them serve that need," says Baulm, 'it's our intention to serve it ourselves."