Among the earliest enthusiasms of Elizabeth Sprague Coolidge when she began to champion the rising composers of her day were the leading Italian musicians of the time. Music by two of these, Pizzetti and Respighi, put a luxuriant sound into yesterday's concert at the Library of Congress.

Called Founder's Day in honor of the great lady who gave the Library and the world of music so much, the concert began with Pizzetti's Epithalamium, a splendid setting of verses by Catullus in celebration of marriage. Scored for solo soprano, tenor, and bartione, the work also uses chamber chorus and orchestra.

Paul Callaway led an ensemble of instrumentalists drawn largely from the National Symphony, with a chamber chorus of superb singers. His soloists were Judith Raskin, Asnastsios Vernios, and Thomas Machen. Pizzetti, one of the moderates in Italian writing in the first half of this century, gave the Catullan lines a sensuous flavor both in orchestra sound, and in the shape of the solo lines, dividing them with a processional refrain as the young wedding party drew nearer.

After Ottorino Respighi's opulent Botticelli Triptych, where the music matches the richness of the Renaissance paintings, the chamber chorus and orchestra took up Ned Rorem's Letters From Paris."

These are at once wholly American, and yet, to any Paris lover, intensely nostalgic with comments about the sex of French automobiles, the French telephone system, and the changing seasons along the Seine. The words come from the letters Janet Flanner wrote to the New Yorker Magazine under the pen name of Genet.

Rorem's orchestra includes piano, harp, celeste, harmonium, saxophone, and much else, all for individual touches. The choral writing is a delight of textures and rhythms. More than once the audience, which greeted the composer warmly at the end of the concert, showed its pleasure at special moments. The concert will be repeated tonight and broadcast at a later date over station WETA-FM.