THE Inter-American Music Festivals reach back farther into Washington's musical history than you might think just by reading that the 12th festival will take place here between May 8 and 14.

Initiated here by Guillermo Espinosa when he was director of the music for the Pan American Union, now known as the Organization of American States, the festivals began over 25 years ago. They are presented at irregular intervals to bring to Washington some of the significant music of the Western Hemisphere in performances by leading musicians of the Americas. A single example will indicate the part these festivals have played in the history of contemporary music. Some of the greatest works by Argentina's foremost composer, Alberto Ginastera, had their premieres at the earlier Inter-American Festivals. His piano concerto, the Cantata Para Una America Magic, his second string quartet, and the cantata, "Bomarzo," which led to the writing of his opera of that name, were all given premieres here in these festivals.

Other memorable scores were introduced by Chile's Becerra-Schmidt and Juan Orrego-Salas, Brazil's Camargo Guarnieri and such legendary composers as Heitor Villa-Lobos and Carlos Chavez.

Following Espinosa's retirement, the direction of the festivals was taken over by Harold Boxer, who has announced for this festival six concerts to be given in the Library of Congress, the Hall of the Americas at the OAS, and the Kennedy Center Concert Hall.

A new festival chorus, trained and directed by Paul Traver of Maryland University, will open the festival at 8 p.m. on Friday, May 8. Singing in the Library of Congress, their program will include works by north and south Americans.

Another newcomer will be Guillermo Scarabino, an Argentine conductor, making his U.S. debut conducting the Festival Orchestra in the Kennedy Center on May 10 with Preludios by Alfonso Letelier, the Symphony No. 2 by Lester Trimble, and the Discovery of Brazil by Villa-Lobos. That concert will also have a major novelty in the U.S. premiere of a concerto for bandoneon, strings and percussion by Astor Piazzola.

The bandoneon is a popular instrument in Argentina, where it is most often heard in popular and folk music. Pablo Casals once said, "Everything can be played on the bandoneon." Piazzola, the composer of the concerto, is widely regardedas the world's foremost performer on the instrument.

Jorge Mester, who is music director of the Aspen, Colo., summer music center, will conduct the Festival Orchestra, on May 11 in the Kennedy Center. His program will include William Schuman's violin concerto, with Ruben Gonzalez as soloist, the fourth of the Bachianas Brasileiras by Villa-Lobos, a tone poem called "Air and Earth," by Luis Santonio Variants by Maximo Flugelman of Argentina.

The Mexico Philharmonic Orchestra is appearing in the festival under the patronage of Madame Lopez Portillo, the wife of the president of Mexico. On May 12 they will play in the Kennedy Center under the direction of Fernando Lozano. After music by Revueltas and Bernal-Jiminez, they will make a strange and unhappy departure from past festival procedures to play Valse Triste by Sibelius and The Pines of Rome by Respighi, neither of which could be said to need performances these days.

There will be two concerts of chamber music inthe festival. On May 9, guitarist Carlos Barbosa-Lima will play in the Hall of the Americas in a program that will include the Concerto for guitar and string quartet by Guido Santarsolo of Uruguay. That same program will also present guitarist Larry Snitzler and flutist Ruth Vinciguerra. The final program on May 14 will bring in the Interlochen Arts Academy Trio, from the famous Michigan music center. With flute, clarinet, and piano, they will play music by Richard Faith of Canada, Halsey Stevens, Roger Sessions, Leonard Bernstein, and Peter Nagy-Farkas of this country, Orrego-Salas of Chile, and Villa-Lobos of Brazil.

All of the festival concerts are free, with seats on a first-come, first-served basis. Concerts in the Kennedy Center are at 8:30 p.m., the others at 8.