WRITING new music for old movies has become a small but vibrant industry. What's interesting about the trend is not only the growing number of popular works emerging from it, but also the message it sends about modern-day moviegoers - who show an increasing taste for film entertainment that includes live performance, with its attendant spontaneity and immediacy, in addition to the prefabricated images of cinema itself. Attaching freshly composed scores to existing films is not a new practice; it happened in silent-movie days, when live music was part of the show. But its renewed popularity in the past 15 years or so is surprising, since audiences have long grown accustomed to music recorded directly on the soundtrack of the picture. The resurgence of live accompaniments gathered steam when Abel Gance's 1927 epic ''Napoleon'' was reissued in 1981, with nostalgically tinged Carmine Coppola music composed for the occasion. Coppola conducted his score in a series of performances, then added it to prints of the film so audiences beyond the concert circuit could enjoy it. A different sort of experiment came in 1984, when a rerelease of Fritz Lang's monumental ''Metropolis'' fleshed out the 1926 fantasy with Giorgio Moroder's new-fangled electronic sounds. Some critics complained that Moroder's highly contemporary music - or noise, depending on your taste - was out of keeping with Lang's vision of a Machine Age gone berserk. More flexible reviewers suggested that Lang and Moroder are brothers under the skin, linked by a shared taste for weird effects. In any case, different versions with either Moroder's score or more traditional music are available on video, so viewers can decide. Philip Glass meets Cocteau Perhaps the most radical of recent movie-music experiments is composer Philip Glass's opera ''La Belle et la Bete,'' which uses Jean Cocteau's film as its main visual component. In this case, the film isn't a silent picture but a 1946 sound production, from which Glass - rarely known to avoid risks - has removed the entire soundtrack, substituting live performers singing the dialogue to his own chamber-ensemble accompaniment. When staged, the production isn't completely fair to the film, which loses its original voice and must suffer the indignity of having highly visible musicians upstage its visual accomplishments. The opera remains an audacious and often fascinating venture, though, and its two-CD soundtrack recording (on the Nonesuch label) is steadily crisp, clear, and evocative. Bill Frisell, another composer who finds inspiration in the movie world, has recently scored three of Buster Keaton's classic comedies (also on Nonesuch): ''The High Sign'' and ''One Week,'' which are major short films, and ''Go West,'' a 1925 feature. Played by Frisell's own group, these quirky scores fall squarely into the jazz-rock ''fusion'' style - too postmodern for ideal compatibility with Keaton's inspired classicism, but offbeat and energetic throughout. Club Foot Orchestra earns renown No survey of the movie-music field would be complete without mention of the Club Foot Orchestra, a unique organization named after the San Francisco nightclub where its members first met. This ensemble has a wide enough range to deal with a wild comedy like Keaton's ingenious ''Sherlock Jr.'' (available from Kino Video) and deeply serious dramas like Jean Renoir's 1926 ''Nana'' and, yes, Lang's ever-popular ''Metropolis,'' among other features and shorts. Its speciality is German Expressionist movies like ''Metropolis,'' the eerie ''Cabinet of Dr. Caligari,'' and ''Nosferatu,'' the first Dracula picture. The orchestra's latest hit is G.W. Pabst's 1928 ''Pandora's Box,'' which it recently performed in a film-plus-music event at Lincoln Center here. The movie oozes with Expressionism, telling the brooding story of a young woman who goes from party-girl frivolity to an unhappy demise in the London fog. Louise Brooks's performance has been praised by countless critics, and Pabst's filmmaking makes stunning use of lighting, editing, and camera work. Much as a film is stitched together from a variety of different shots, the Club Foot Orchestra assembles its music from contributions by each of its members, who work on different sections of the movie being scored. As unwieldy as this method sounds, it's capable of producing polished and coherent results, as the ''Pandora's Box'' music proves by embellishing the power of Pabst's film while providing substantial artistic interest of its own. The score's idioms fall into familiar patterns at times - echoes of Kurt Weill, in particular, are too compelling for some of the group's composers to resist - and some portions are stronger than others. All told, however, the concert version of ''Pandora's Box'' is a splendid specimen of its increasingly popular breed. One looks forward to further adventures of this hardy group, but they won't be coming until a current project is completed: music for a 13-part cartoon series based on the immortal ''Felix the Cat'' animations of the silent-film era. They start airing on CBS next month.