LEONARD SLATKIN LEADS THE MINNESOTA ORCHESTRA July 15 for the opening concert of the Sommerfest Music Festival in Minneapolis. Entitled ``Vienna and the Romantic Tradition,'' the festival offers music of Schumann Schubert, Brahms, Mozart, and Beethoven, plus the debut of an orchestral work by Kamran Ince. The eighth season closes Aug. 8 with a concert production of Verdi's ``Otello,'' starring James McCracken in the title role. THE GRATEFUL DEAD TODAY RELEASES ITS FIRST ALBUM in seven years, called ``In the Dark.'' Remaining dates of the band's US tour are Philadelphia (Friday), East Rutherford, N.J. (Sunday), Eugene, Ore. (July 19), Oakland, Calif. (July 24), and Anaheim, Calif. (July 26). An hourlong concert video is projected for December release. MOZART'S OPERA ``COS`I FAN TUTTE'' opens tonight at the Alfresco Festival of Lyric Opera Cleveland. Michael McConnell directs this new production, the first in a five-year cycle of Mozart operas. The world premi`ere of Larry Baker's ``Haydn Head'' double-bills with Menotti's ``The Medium'' on July 30. Festival ends Aug. 8.