A heady mix of sounds -- the music of Philip Glass

If you haven't run across Philip Glass's music yet, look out. It could be waiting around the next corner. Although many have dubbed his style too radical for popularity - with its repetitive melodies and insistent rhythms - Glass's unusual sounds are finding a surprisingly wide audience. His new CBS Masterworks record, ''The Photographer'' (FM 37849), is selling even more briskly than his earlier CBS disc, ''Glassworks ,'' which did phenomenally well for a classical release.

Just returned from a far-flung American tour, Glass will give a Carnegie Hall concert tomorrow - his last appearance there sold out the house - and a European tour begins in November. The full theatrical version of ''The Photographer'' (including a concert, a play, and a dance) will have its American premiere this fall at the Brooklyn Academy of Music, and will tour the United States next spring. Glass's newest opera, ''Akhenaton,'' bows next March in West Germany, to be followed by American productions in Houston and at the prestigious New York City Opera.

Still in the talking stage are a revival of his opera ''Einstein on the Beach ,'' at the Brooklyn Academy; a CBS recording of the opera ''Satyagraha''; and reissue of various early Glass discs on the Masterworks label.

He's everywhere!

What's behind this emerging success story? I put the question to Glass during a talk at his modest Manhattan apartment, as pictures of Rossini and Sibelius gazed at us from a perch on the piano.

Part of the secret, he answered, is his own hard work on behalf of his music. In the tradition of concertizing composers, he has performed his own pieces in public for nearly 15 years. He has published and recorded his compositions through his own organizations. Determined to bring the music before a broad public, he formed the Philip Glass Ensemble and played wherever people would listen. And he never hesitated to break the ''rules'' of the musical game - appearing in rock clubs as well as concert halls, for example, if that's where enthusiastic listeners were to be found.

This much said, Glass paused and laughed at himself for jumping straight to the ''practical'' aspects of his career. ''The actual music is important, too,'' he went on.

For years, that music was written specifically for his ensemble, with its unusual blend of woodwind and keyboard instruments. Now, as his prestige - and funding - have risen, Glass has turned to more orchestral forces. ''The Photographer,'' for example, includes brass, strings, reeds, keyboards, a chorus , and a synthesizer.

''I've wanted to use many different instruments before,'' Glass reports, ''but it wasn't feasible. This time I could get enough of a budget from CBS, which cleared the way. So much of it comes down to money!'' he concludes with a bemused sigh.

Not everyone is happy with the newest directions in Glass's career. With its steady pulse and assertive textures, his earlier music - such as ''Einstein on the Beach'' and ''Dance'' - evokes not only the sophistication of classical traditions but the audacity of jazz and the energy of good rock, along with the cyclical structure of Eastern music that Glass has long admired. His latest pieces grow directly out of this work, but their orchestrations seem less bold and their melodic patterns less surprising - though ''The Photographer'' is spiced with biting harmonies and unique combinations of acoustic and electronic instruments.

Glass freely acknowledges that he wrote his recent ''Glassworks'' in a consciously conservative style, partly to encourage the confidence of CBS, which had recently made him the third composer (along with Igor Stravinsky and Aaron Copland) to sign an exclusive Masterworks recording contract.

''The Photographer'' finds Glass heading toward fresher territory again. The work was written for a theater piece about Eadweard Muybridge, a real-life pioneer of modern photography who was tried for murdering his wife's lover in 1874. ''His life was dramatic, even melodramatic,'' says Glass. ''And all my pieces grow out of their subject matter. So this music is emotional. Even the harmonies express the great drama that surrounded Muybridge.''

Also on the Carnegie Hall program will be a work not yet recorded, taken from Glass's score for the film ''Koyaanisqatsi,'' a plotless examination of contemporary Western foibles. It combines the orchestral textures of ''The Photographer'' with the tunefulness of ''Glassworks,'' marking another step away from the unique sound of the unadorned Glass ensemble.

''The ensemble is still the core,'' Glass says, ''and I still like the voice we've developed by playing together for 14 years. But the ensemble has also been a bridge to other kinds of writing. You won't hear its influence at all in 'Akhenaton' - though it's taken me five years to work my way out of the old sound.''

Glass's career is nothing if not busy. Besides composing and performing, he participates in ''marketing the product,'' as he jokingly puts it, and makes practical decisions. For example, he must continually calculate how much he's willing to use his own personality in promoting his work - agreeing to yet another round of in-person performances, perhaps, but refusing to have his photo on a record jacket.

''I've had to be resourceful to get my music played and listened to as widely as possible,'' he says. ''You don't always hear about this kind of thing, because in the classical music world there's a lot of pressure toward a false dignity - an attitude that we're above all this. But some of us aren't above it at all!'' he admits with a cheerful smile.

Despite the distractions of bringing his music to its audience, Glass gets an amazing amount of composing done. In the past 12 months he has written ''The Photographer,'' almost finished the 31/2-hour ''Akhenaton,'' written a song, composed music for two shows by the Mabou Mines theater group - and still managed to play 30 concerts and spend time with his children.

How does he do it? ''I've been composing since I was 15,'' he answers. ''The first five years were very hard. The next 10 were easier, but I still wrote slowly. Then, when I was about 30, I began to acquire an ease. Maybe it's just that I learned not to have anxiety about it. In any case, I write fast. But there's nothing wrong with that. It has nothing to do with being careless.''

To prove his point about being careful, he shows me a few pages of the ''Akhenaton'' manuscript, proudly noting how few erasures there are. ''The thing is,'' he goes on, ''I think about music all the time - to the annoyance of my family and friends. I dream about music! So the actual notating of a piece may not take any time at all. The process of writing it down doesn't reflect the actual creation of the work. . . .''