For the last 30 years or so American jazz fans have had little trouble cozying up to Brazilian vocalists, no matter how thin their jazz credentials. Even Brazil's pop music has so much of what jazz listeners seek, in terms of rhythmic complexity and harmonic imagination, that it can carry many singers past their own weaknesses. As a result, we tend to shrug off the idea of a true jazz singer from Brazil--but that's where Leny Andrade comes in. She sings in a powerful, burred contralto--husky almost to the point of androgyny--and she seems constitutionally bound to impart the deepest swing to everything from bossa nova standards to American jazz tunes. (This rhythmic authority allows her to slow down a song like "A Night in Tunisia" to quietly startling effect.) And she improvises, her scat work occupying a middle ground between the creative vitality of Ella Fitzgerald and the swaying melodies of her native land. Andrade has earned the sobriquet "the first lady of Brazilian jazz," but listeners in this country have had little chance to hear why: her handful of albums don't do justice to her reportedly exuberant live performances. Yet even on these she emerges as a singer of unquestioned skill and provocative artistic integrity. Andrade's appearance--believed to be her Chicago-area debut--is part of Ravinia's "Musica Viva" series, cosponsored by WBEZ FM, which employs this writer. Wednesday, 7:30 PM, Bennett Hall, Ravinia Festival, Green Bay and Lake Cook Rds., Highland Park; 728-4642.