Dale Clevenger, the first chair of the Chicago Symphony Orchestra's fabled horn section, is arguably the busiest musician in town. Not only is he chiefly responsible for creating the mellow, sensuous sound that has made the orchestra a powerhouse in the late 19th-century repertoire, but his mellifluous horn playing can also be heard in a score of commercial jingles. Clevenger doesn't solo as much as he ought to--those lucrative commercial contracts con keep a muscian very busy--but when he does it's a special treat for horn fanciers in particular. For this concert he's scheduled pieces by two composers who really knew how to compose for the horn. Mozart wrote Horn Concerto no. 1 in 1782, shortly after moving to Vienna. It's a delightful confection, treating the horn as an easygoing, festive voice. In Richard Strauss's Andante for Horn and Piano the voice is much more weighty, dramatic, sonorous. Beethoven's Sonata for Horn and Piano, also on the program, is a skillfully designed exercise, but it's not as substantial as his other sonatas. A promised bonus: Clevenger as jazz musician, performing works by Duke Ellington. His "friends" in this showy affair are some of his colleagues in the orchestra, Plus, in solo roles, Daniel Barenboim at the piano and harpist Edward Druzinsky. Cliff Colnot conducts. Monday, 8 PM, Orchestra Hall, 220 S. Michigan; 435-6666 or 435-8122.