"THIS SUMMER," says Bill Lockwood, "we are trying a few pieces by Salieri . . . But he is not box office -- not the way Mozart is box office."

There is a heavy irony in the words of the director and co-founder of the Mostly Mozart Festival. If Mozart could see what has happened to his music in the last 200 years, he would be amazed. In his lifetime, Antonio Salieri was a composer enormously more respected and admired than Wolfgang Amadeus Mozart. Today, the music of Salieri can find an audience only as a curiosity in a program dedicated to the music of Mozart -- and only because the Mozart-Salieri rivalry is the subject of a hot show on Broadway. Overshadowed by Salieri's in the 18th century, Mozart's music was outranked by Beethoven's in the 19th and most of the 20th. Now, aided by long-playing records, the pioneering work of such conductors as George Szell, and the development of a younger generation with a new taste, Mozart is at the top of the classical heap, attracting a T-shirt-wearing audience that seldom goes to concerts in establishment showcases like the Lincoln Center and the Kennedy Center. "It's an audience that we saw at Lincoln Center only at film festivals, not at concerts," says Lockwood. "Our surveys indicate that about 40 percent of our audience is under 35 and a good 20 percent under 25. Everyone wears shoes, because the law requires it, but you don't see any neckties."

Lockwood is wearing a T-shirt emblazoned with a New Yorker cartoon captioned "Life Without Mozart": a desert landscape littered with an empty tin can, an empty bottle, an abandoned automobile tire. But, being the director of programming for Lincoln Center on a more or less official visit to the Kennedy Center, he conceals his T-shirt under a regular shirt, necktie and business suit.

Mostly Mozart opens its five-day run in Washington on Tuesday. After 15 smashing summer seasons in New York, this is the first time Lockwood has taken the show out of town, though there is a clone festival on the West Coast, owned and operated by the San Francisco Symphony with the blessing of the originators.

This venture into new territory seems to worry Lockwood not at all. He is in charge of one of the most demanded and expertly marketed commodities in classical music. Last season in New York, Mostly Mozart sold 95,000 out of a possible 96,000 seats -- not to mention T-shirts, mugs, tote bags, aprons, pencils, bookmarks, coasters and beach towels. Its basic subject, Mozart, is one of the greatest creative artists in history and -- more important -- one who generates a special kind of affection.

Undoubtedly it helps that the slow movement of a Mozart piano concerto was used in a movie soundtrack and is now nicknamed "Elvira Madigan." And it helps that Mozart is the subject of "Amadeus" -- a play that has settled in for a long run on Broadway and won five Tonys this season. But that hardly seems necessary. It is certainly pleasant that the festival's first run at the Kennedy Center will include the American premiere of a newly discovered symphony from Mozart's childhood, but that is merely the whipped cream on the cake.

It is gratifying that the festival each year includes such international superstars as Jean-Pierre Rampal, Alicia de Larrocha and Pinchas Zukerman at a fraction of their regular fees -- because they like the festival and because it gave them exposure before they had reached their present giddy eminence. But the key fact of the Mostly Mozart Festival is that it is not "about" star performers; it is about the music -- about Mozart. Or, as the title indicates, primarily about Mozart. "Occasionally, another composer slips in the back door," says Lockwood, "and we look the other way. We're trying one Brahms piece this summer, but it's usually just Mozart with a bit of Beethoven, Haydn, Schubert and Bach. We tried a Stravinsky piece once, but we got letters of complaint."

Until Mozart's lifetime, practically the only music people heard (except possibly in church) was contemporary music. The formation of a canon of classical music began during the late 18th century, partly because of King George III's affection for the music of Handel and Baron Von Swieten's reverence for the music of Bach, composers of a previous generation. Mozart participated in this process, reorchestrating Handel's "Messiah" and transcribing some of Bach's keyboard music. After these two, he was (with Haydn) one of the first composers to reach blockbuster classic status, but it happened slightly too late to do him any good. If he had lived to be 77, as Haydn did, he might have enjoyed the kind of glory and adulation Aaron Copland is enjoying today. But he died (in 1791) at age 35 and in abject poverty.

In his lifetime, Mozart was known chiefly as a piano virtuoso; much of his music evoked enthusiasm only in a few connoisseurs, and he was unable to get and hold the kind of steady job that might have prolonged his life. Today, by most measurable standards, he is the most popular of all composers, though some of his music is almost never heard except on records or in specialized contexts such as the Mostly Mozart Festival.

Last year, in the Schwann Record and Tape Guide, which lists all classical records normally available through American record shops, Mozart had 112 new listings, taking first place ahead of his two perennial rivals for the position, Beethoven (who had 102) and Bach (96). This month, according to the composer index of Forecast magazine, compositions of Mozart will be broadcast 111 times on good-music stations in the Washington-Baltimore area. Some popular pieces such as the Clarinet Concerto, the 17th Piano Concerto and (inexplicably) the Duo in G for violin and viola, will be broadcast three times on various stations -- though Eine Kleine Nacthmusick, perhaps his best-known piece, will have only two airings. In comparison, FM fans will be able to tune in 53 times to J. S. Bach, 66 times to Beethoven.

The Forecast listings are not quite complete -- for one thing, they omit the programming of WGMS -- but they are certainly a fair index of what happens on the air. Paul Teare, program manager of WGMS, confides that his station, too, plays more of Mozart than of anyone else. "Part of the reason," he says, "is that there is so much of it. We try not to program any major work more than once a month."

The abundance of Mozart's music -- as well as its quality and variety -- explains why he is one of the few composers who can be the primary subject of a month long festival. In the Koechel listing, which has been the standard catalogue of Mazart's works for a century, the Requiem, his last, unfinished composition, is listed as no. 626 -- but that gives no accurate idea of the number of his works. In the revisions of the catalogue through the past century, many of the numbers have been subdivided -- like K. 61d or K. 73t, with a letter of the alphabet added to make one number serve the purposes of many. And even within a single number, there can be more than one work; K. 61d, for example, is a set of 19 minuets for small orchestra.

The music's variety ranges from operas, serious and comic, symphonies, concertos, serenades, string quartets and quintets through a dazzling variety of church music, little piano pieces, songs that launched the German Lieder tradition, and obscene canons. Mozart composed for every performing medium available in his time, including mechanical organ and a strange instrument called the glass harmonica. When Lincoln Center began looking for a composer whose work was abundant, varied and charismatic, the choice was inevitable.

"It started as all Mozart," Lockwood recalls. "We called it 'Midsummer Serenade: A Mozart Festival,' and the idea was to do something new in summertime to fill the concert hall, while the New York Philharmonic was on holiday or playing in the parks. It was Lincoln Center's responsibility to come up with something if it wanted to keep its doors open."

The idea originated in a brainstorming session with William Schuman, who was then the president of Lincoln Center, Schuyler Chapin and Lockwood. Suppose you took one composer and did a whole month of concerts devoted to nothing but his music, they wondered. "When we thought about it," Lockwood says, "it was quite obvious that Mozart was the only composer who fit the bill in terms of writing for every conceivable combination -- choral, orchestral, chamber music and solo music for recitals -- and also one who could fit within our budget -- music for chamber orchestra. We couldn't afford a 100-piece orchestra to do a Brahms festival. Mozart didn't write for more than 40 players -- ever.

"It also had to be someone whose music we could inflict on the public, night after night, six nights a week, and Mozart is about the only composer who wears that well because of his infinite variety -- so he was the obvious answer. The first two seasons were all Mozart, and we didn't repeat a single work for two years. In 1968, we let in Haydn on sufferance, and in 1970, we added Schubert. At that point, we looked at the schedule of concerts and it was about 50 percent Mozart land 50 percent Haydn and Schubert. I said one day . . . 'These concerts are really mostly Mozart. Why don't we call it that? Why don't we drop the Midsummer Serenade thing, which is sort of corny, and call it Mostly Mozart; give it a name and maybe it will catch on.'"

And indeed it has. The project begun to help keep Lincoln Center open in the dog days has become one of its most popular annual activities, even on nights when it is competing with a free concert by the New York Philharmonic. "When they have an audience of 150,000 in a park," Lockwood says happily, "we're still sold out."