In this year's American Music Festival, which began last night at the National Gallery, we will be hearing more music than usual by lesser-known composers, most of it written in this century. The nine Sunday concerts will bring a variety of chamber ensembles and soloists to the East Garden Court and, as usual, will be highlighted by several concerts by the National Gallery Orchestra under Richard Bales.

For the opening concert, Bales paid homage to the early Americans he has espoused for so long, then set the stage for the rest of the series by playing two works by Wayne Barlow: a first Washington performance of "Images" for harp and instruments with Robert Barlow, the composer's son, as soloist, and a "Sinfonia da Camera."

These pieces were written in the early '60s, but sound sort of fortyish. They are amiable works. They chug along with academic predictability, relying on lush sonorities and reliable formulas to fill the void left by an absence of solid musical ideas.

Robert Barlow gave a secure account of the demanding solo part of "Images." He has a nice repertoire of timbres and a fine, even touch.

For the first half of the concert, Bales put his baton where his heart is, at the service of music from the revolutionary period. With the e'lan that always characterizes his performances of such music, he gave delightful readings of his orchestra arrangements of "Music of the American Revolution," three songs of early America, and "The Battle of Trenton--a Favorite Historical Military Sonata, for the Piano, dedicated to George Washington" by James Hewett. It was a fine way to celebrate Washington's 250th birthday.