The 20-member Cathedral Singers, a youthful choir founded four years ago by composer Richard Proulx to record a wide variety of choral music, has already released ten CDs on the Canticum label that demonstrate a versatility and assurance usually associated with veteran ensembles. The fluidity and clarity of the group's singing can also be heard on its latest CD, Baroque Music From the Bolivian Rainforest (on Tech), a typical theme-oriented compilation. Two motets from this anthology are on the bill of the opener of the Cathedral Singers' new occasional series. Both are curiosities from the surviving manuscripts of vesper pieces written in the 17th and 18th centuries by anonymous European and local musicians in Jesuit missions in Bolivia and intended to help convert the natives to Catholicism. Most of the vocal and instrumental works in the archives of the Bolivian town where these two works were found were patterned after established Old World models, but the instruments and vocal mannerisms were often of indigenous origin, lending an exotic cast to the music and reinforcing the church's rhetoric of inclusion. Proulx's taste veers toward the Anglican, but the focus of this generous program seems to be the variety of modern expressions of worship, with entries from John Tavener ("Angels"), David Hurd ("Awake, Awake"), Peter Hallock ("The Lord Is My Light"), and William Mathias (Toccato Giocoso for organ). Proulx will conduct. Sunday, 7:30 PM, St. Luke's Evangelical Lutheran Church, 1500 W. Belmont; 973-3213.