The toe-tapping Broadway musical, with its big, brassy orchestra and knockout chorus line, faces a bleak future, according to a report released yesterday by the National Endowment for the Arts.

"The musical theater has an absolute crisis created by the general drying up of its traditional talent base and production economy," writes Robert Marx, former director of the New York State Council on the Arts Theater Program. During his presentation yesterday to the National Council on the Arts here, Marx fielded questions about his study.

Marx, who holds a doctorate from the Yale School of Drama, spent about six months traveling the country to interview 80 of the top figures in opera, musical theater and experimental theater. He received a $40,000 commission to gauge the current state of opera, musical theater and experimental theater in the nation.

Among his findings:

*Opera companies rarely develop new works because they are run by administrators who prefer known box-office quantities. "As a very broad generalization, creative artists, when given the reins of an institution, tend to be more open to new work and take chances on it."

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*Experimental theater, or music theater, which combines music, dance and alternative theater, "is now producing and communicating new musical styles in the theater to a large audience that is willing to support it. it." The art form, Marx writes, "is moving into the national center."

*Broadway cannot support a community of artists as it did from World War I through the 1950s. Composers, librettists, directors, lyricists and performers have fled to the movies and television. "No American play of any international stature, critical significance or seriousness has emerged from the Broadway theater" since "Who's Afraid of Virginia Woolf?" in 1962, according to the report.

The presidentially appointed National Council on the Arts, which advises the NEA, generally praised Marx's report, though some voiced minor criticisms at yesterday's meeting. Conductor and opera producer Kurt Herbert Adler found it too pessimistic about the fate of new operas and musicals.

Dance director and choreographer Martha Graham urged the council to concentrate on nurturing individuals, rather than companies, to create new works. Actress Celeste Holm voiced a similar opinion, arguing that young minds must be inspired to create for the theater. "Unless schoolteachers take them to the theater . . . beyond the shallowness of TV, they're never going to get anywhere," she later said.

When the focus turned to the fostering of new operatic works, critic/publisher Samuel Lipman said opera companies have been moving in the wrong direction, away from important works in favor of entertainment.

In his report, Marx notes the increase in opera companies performing musical theater. Although many performances have been successful, artists are still undecided about whether musicals belong in opera houses.

At the end of the discussion, NEA chairman Frank Hodsoll said he didn't know the answers to the current dilemmas facing musical theater and opera, but said he expects to produce a report within six months proposing some initiatives.

One goal will be to pinpoint the musical theaters that can introduce new works. The report states that opera houses are too big and regional theaters too small, while theaters seating between 800 and 1,000 appear ideal for new works.

Another unresolved question is how to encourage the development of new operas. The report says composers need guidance to write music for the stage and enough rehearsal time to hone their work. According to Marx, most of the great operatic composers wrote many flops before achieving success, a fact that must make today's financially conservative opera managers somewhat queasy.

The council will also examine ways to sponsor experimental theater. A recent revival of Philip Glass' opera, "Einstein at the Beach," cost more than $1 million to stage.

No one, including Marx, offered any solution for Broadway's woes. "I think we've lost it," Marx said later. "I hope we've lost it only temporarily."