Mike Mills sold over 60 million albums as bassist and songwriter with R.E.M., which emerged from the college-rock scene of the 1980’s to become one of the world’s most successful bands. Now he’s blending genres with his concerto for orchestra and rock band. Mills and violinist Robert McDuffie will perform it Friday with the St. Louis Symphony Orchestra, in a program that also includes orchestral interpretations of R.E.M. songs.