You may not be able to hear it, but the streets are alive with the sound of Sonys.

On M St. the roller skaters, in Rock Creek Park the joggers, on Wisconsin Ave. the browsers -- all listening to stereo headphones wired to a pocket-sized cassette player called The Walkman, "the middle and upper class answer to the box," in the words of The Wall Street Journal.

"It's the disease of the '80s," says Gene Pressman, who runs Barney's, a revitalized department store in New York City.

"We just got back from Paris and everybody's wearing them," says Andy Warhol.

"We can't keep them with the demand," says Fred Wahlstrom of Sony, which has unleashed on the U.S. more than 500,000 of the $200 cassette players in the last year or so.

All this madness started about three years agop with a disgruntled corporate leader, Sony's Akio Morita (chairman, CEO and co-founder to be specific), who wanted to be surrounded by stereophonic music as he volleyed on the tennis court -- oblivious, one presumes, to any undesired audio encroachment. Wasn't this something the great scientists at Sony could design? Hadn't Dr. Edwin Land put his armies to work to develop modern-day conveniences like color film and the SX-70 and Polavision instant movies? Better living through chemistry and all that. So Morita summoned his engineers one Monday morning . . .

And lo and behold the Sony scientists created Walkman, a 5-and-5/16-by-3 l/2-by-1-3/16-inch blue aluminum rectangular cassette player powered by two penlite batteries and weighing in at a mere 13 7/8 ounces. The little unit could be strapped to a belt, and out from the Walkman extended a five-foot length of cable terminating in a set of ounce-and-a-half headphones. Voila: portable stero sound, totally programmable by the user, without relying on the uncontrollable impulses of radio broadcasting.

"With the advent of the Sony Walman came the end of meeting people," says Susan Blond, a vice president at CBS Records. "It's like a drug: You put the Walkman on and you blot out the rest of the world."

The Walkman became an immediate hit in Japan in 1979, particularly with visiting foreigners who knew an instant status symbol when they saw one. Vitas Gerulitas and Bjoin Borg picked them up on the tennis circuit and were snapped for the pages of Women's Wear Daily. Movie and music types followed quickly: Britt Ekland, Suzanne Somers, Paul Simon (who wore one at this year's Grammy Awards). By the time Walkman was introduced in the United States early last year, it was a stanard celebrity item. Donna Summer gave them as Christmas presents by the dozen.

Now they've become an everyday part of American culture. There are Walkman used in ads to sell cassettes, 7-Up and Seagram's 7. There are Walkmen in fashion ads. "The best way to get something to catch on," says photographer Francesco Scavullo, himself a user, "is to use it in fashion photographs. The Walkman is like fashion itself: It's a gimmick." There are even unexplained references to the Walkman in news stories, as in this quote from a recent issue of Time magazine.

"Human history is like being a pilgrim to Jerusalem -- two steps forward, one back. And folks worry about technology. Personally, I like it. When I can't sleep, I listen to Bach on my Sony Walkman. That's progress."

"It's a very convenient thing," says musician Todd Rundgren. "Most musicians I know use them. It's the beginning of the tape equipment of the future."

Not to mention practical: "The earphones stop my hair from blowing around," says Warhol. "I use mine all the time to listen to opera. It's nice to hear Pavarotti instead of car horns."

Not to mention an indisputable index of hipness: "I see them a lot in first-class cabins of airplanes and around the pool at the Beverly Hill hotels," says Blond.

Not to mention a surprisingly successful luxury item in the midst of a soft economy. "The response is phenomenal at that price point," says Joanne Stellar of Woodward & Lothrop, which sold hundreds since last fall. "This is not an inexpensive item. It does have super sound quality: You can walk around and hear great music without schlepping a whole stereo system. Maybe it's so popular because nothing like this existed before."

By now, however, many similar products have appeared: Two dozen were shown in prototype at this winter's Consumer Electronics Show, even as Sony exhibited its own new, smaller Walkman II that will be introduced this month. Five other units are already on the market, from the $60 Mura AM/FM radio to the $219 Toshiba, which plays tapes and has a radio as well. aOther manufacturers are Sanyo, Technidyne and Cybernet.

"But everyone wants the Sony so far," says Dave Euley of Reliable Home Appliances in Springfield. "It's the name. The Sony is the status symbol in this product. Everybody has been buying this thing, no age group in particular. I was in Colorado for two weeks this winter and they were renting them on the slopes. There were an incredible number of people wearing them. You can bet they all went home and bought them."

Indeed, the Walkman has become so pervasive that it may help define a new style of music.

"Phil Spector used to mix records on a jukebox in a studio when that was the dominant mode of listening," says Mark Leviton, an executive at Warner Bros. Records. "I was reading a review in a British music paper the other day, and it said, 'Perfect mix for a Walkman.'"