Perhaps you've already heard about this group, whose recent LP release, Le Mystere des Voix Bulgares (why a French title, I wonder) not only knocked critics out all over Western Europe and North America, but also sold well enough to have spawned the brand-new volume two. Certainly the technical prowess of these singers is second to none, but they're not coming from the western classical vibrato-oriented tradition you might expect. The basic sound here is much the same medieval nasality one hears in various guises from Sardinia to Saudi Arabia--a guileless, strident, richly emotional vocal throb here refined to a high polish and multiplied by several dozen for spectacular choral effect. The choir performs arrangements of various ancient Eastern European folk songs and chants that start out simply, but explode in a profusion of eerie, heartbreaking harmonic intervals. But what's really amazing is the extent to which these singers manage to modernize and sophisticate the folk material without watering it down. On record, they sound for all the world like the chorus of some prehistoric golden age delivered to us in a time machine--and the thought of hearing them live is intriguing, to say the least. Thursday, 8 PM, Park West, 322 W. Armitage; 929-5959.