After a prolonged absence, Soviet musicians are once again being allowed to concertize in the U.S. The impressive parade of Russian performers into Chicago began last summer with a recital by the Taneyev Quartet; it continues this week with the appearance of the Moscow Virtuosi. Founded in 1979 by violinist Vladimir Spivakov and consisting of top-ranking instrumentalists, the Virtuosi are hailed for their impeccable musicianship, noble taste, and joie de vivre--qualities that have defined the finest chamber orchestras of the past. It will be of particular interest to see how they handle the pair of elusively Slavic works on the program: the sprightly Concerto for Piano, Trumpet, and Strings by Shostakovich and Tchaikovsy's enchanting Serenade for Strings: will they sound more "Russian"? Soloists include pianist Vladimir Krainev, a Tchaikovsky Competition gold medalist, American trumpeter Stephen Burns, and Spivakov himself, a virtuoso said to be in the tradition of the great David Oistrakh. Monday, 8 PM, Civic Theatre, 20 N. Wacker; 242-6237.