For years, the Detroit Symphony Orchestra has produced large-scale choral works with the assistance of a polished adult choir based in nearby Ann Arbor, Michigan. This year, however, that choir is absent from the DSO’s schedule, and, in an apparent cost-cutting move, the orchestra is putting on performances of Mozart’s Requiem with a 60-voice student chorus from a state university. The result, says critic Lawrence Johnson, is embarrassingly bad, and an insult to the DSO’s audiences, which pay good money to hear professional-caliber performances.