Music of the Baroque offers its popular brass and organ concerts this week, pleasurable wallows in loud instrumental polyphony featuring compositions by some of Baroque's most beloved composers. Unfortunately it's too early to say exactly which composers will be featured (though early betting is on Bach, Telemann, and Gabrieli): it is conductor/organist Thomas Wikman's custom to select and rehearse a large number of likely candidates, then put the program together at the lost minute from the best numbers. The brass soloists have been imported from around the country: trombonists Mark Lawrence of the San Francisco Symphony, Michael Mulcahy of the CSO, and Max Bonecutter of the University of Kansas, and trumpeters Barbara Butler and Charles Geyer, both of the Eastman School of Music. Whatever they play--last year's concerts included works by Lassus, Purcell, Buxtenhude,- and Praetorius--it should be a good time for those who appreciate tonal intricacies and brass instrumental colors. Wednesday, 8 PM, Alice Millar Mem-orial Chapel, Northwestern University, 1870 Sheridan, Evanston. Next Friday, March 25, 8 PM, St. Clement's Church, 642 W. Deming. 986-3236.