Conductor Richard Bales raised his baton a few minutes after 7 last night in the National Gallery of Art, and the orchestra launched into a potpourri of "Yankee Doodle," "La Marseillaise," "The Irish Washerwoman" and other tunes, familiar and less familiar, that date back to the Federal period. The 40th annual American Music Festival was under way in the vast, echoing marble vault of the East Garden Court. The nine-week event (continuing every Sunday night through May 29) began with a mixture of great music, historical relics, interesting oddities and good, clean fun.

In future weeks, the festival will present singers, solo and choral; chamber ensembles; pianists; and two more appearances by the orchestra. Some composers will be familiar--Ives, Persichetti, Barber and MacDowell, for example. Others (Horatio Parker and Charles Tomlinson Griffes, to name two) have secure niches in American music history but seldom appear on concert programs. There will be some emphasis on composers from the Washington area and a lot of modern music, since most serious American music dates from this century.

There will be more than a dozen world or local premieres, and the festival will end with the first Washington performance of a work that is more than a century old by one of the major names of America's musical past: the Symphony No. 2 ("In Spring") of John Knowles Paine, composed in 1880. Not all of the music will be great (or even will try to be), but there should be many happy discoveries for listeners whose tastes go beyond the standard repertoire.

The opening concert gave a fair sample of the festival's scope. It included two certified masterpieces: Griffes' well-wrought, impressionistic "The Pleasure Dome of Kubla Khan" and Howard Hanson's powerful Symphony No. 1 ("Nordic"). There also were two charming bits of trivia: a Federal potpourri, arranged by Romulus Franceschini, and a lilting dance number called "Hobby on the Green," composed by Hilton Rufty and orchestrated by Richard Bales.

The evening's world premiere was a Suite for Orchestra titled "To Elmira with Love," revised and orchestrated by Bales from a piano suite he composed in 1972. It was dedicted to Elmira Bier, who for many years was the music director of the Phillips Collection concerts--Bales' friendly competitor in the presentation of free music in Washington on Sunday evenings. It uses the letters of her name as the basis for a loose sort of tone row, which Bales wrests into a polka, sarabande and toccata as well as various mutations of itself. The element of fun, even of parody, is emphasized in its orchestral form, with lots of brass and percussion to spice up the sound, and deft applications of shimmering string tone for wistful moments. It received a loving performance, as any composer's music does when Bales holds the baton.

The acoustics of the East Garden Court are, of course, problematic and never more so than when the music is orchestral. Soft passages, particularly in the woodwinds, seem least subject to reverberation in this hall, but the music begins to acquire a sort of halo as it gets louder, and very loud passages, particularly in the bass and percussion, tend to engulf the hall, to immerse the listener in massive waves of sound. These effects sometimes detracted from the clarity of last night's performance, but in a way enhanced the power of climactic moments in the Hanson and added a sort of impressionistic shimmer to the Griffes orchestration.