Labels such as "world-class" and "sine qua non" have been applied--all too often indiscriminately--to the Chicago Symphony under Georg Solti when they are more appropriate, in my opinion, for a couple of the CSO's satellite chamber groups. Take the Chicago Pro Musica: this bunch of world-class instrumentalists seems to get a charge out of playing music few concertgoers have ever heard of, and their special zeal for widening the boundaries of the mainstream 20th-century repertory is truly sine qua non. Under the avid guidance of clarinetist John Bruce Yeh, who's up on all the latest recording techniques, the Pro Musica has won a well-deserved Grammy and continues to record worthwhile (though neglected) pieces that call for unorthodox instrumental combos. I suspect the works by Prokofiev in this tribute to the great Soviet composer on the centenary of his birth--performed by the Pro Musica in Leningrad last fall--will be out on CD in the near future. They are the Quintet for Oboe, Clarinet, Violin, Viola, and Bass op. 39 and "Duels and Dances," from his crowd-pleasing 1935 ballet Romeo and Juliet (in an arrangement written for the Pro Musica. by William Neil, Lyric Opera's erstwhile composer in residence). An added attraction is Yuri Falik's English Divertissement for Flute, Clarinet, and Bassoon (1978)--whose title bespeaks the conservative taste of the composer, a contemporary Russian of limited renown. Sunday, 7 PM, ballroom, Orchestra Hall, 220 S. Michigan; 435-6666 or 435-8122.