Pierre Boulez is a relentless advocate of 20th-century music, but due in part to his habit of immersing himself in it one composer at a time–as he’s done recently with the likes of Bartok, Mahler, and Scriabin–he’s only now turning his attention to Leos Janacek. Born in Moravia in 1854, Janacek is often lumped in with Dvorak and Smetana as a Romantic and central European nationalist, fond of folk melodies, and in fact that was a fair way to characterize him for much of his early career. But by the time he composed his most significant pieces, beginning with the opera Jenufa (1903), he’d developed an idiosyncratic style that would place him in the company of Berg, Schoenberg, and Stravinksy. Its cornerstone, “speech-melody,” was a method of patterning his melodies after the jagged, fragmentary shapes and explosive rhythms of Czech vocal exclamations–he often jotted down phrases he overheard on the street, then attempted to re-create in music their wildly disparate moods. His instrumental lines cover the entire dynamic and expressive range of human utterance, from a deep, throaty drone or barely audible hiss to skull-splitting shrieks and screams, and when he wrote for voices the results were both frighteningly violent and powerfully moving. One of the most vivid examples of this technique is the Glagolitic Mass (1926), which the Chicago Symphony Orchestra will perform here under Boulez’s baton. Janacek was agnostic, so it seems likely that his desire to compose sacred music himself arose from his admiration for Beethoven’s Missa Solemnis; true to his roots, though, he wrote his mass not in Latin but in an old liturgical Slavonic language. Earlier versions had as few as five movements, but the CSO is using a new reconstruction of the original score that includes all seven, restoring material Janacek cut after an inadequate ensemble botched the mass’s premiere. Soloists include soprano Elzbieta Szmytka, mezzo Nancy Maultsby, tenor Stuart Neill, and bass-baritone Nathan Berg. Rounding out the program are two Stravinsky pieces, Symphonies of Wind Instruments and the Symphony of Psalms. Thursday and Saturday, November 2 and 4, 8 PM, and Friday, November 3, 1:30 PM, Orchestra Hall, Symphony Center, 220 S. Michigan; 312-294-3000 or 800-223-7114.