Music matters for choreographer Mark Morris. He insists on having live music during dance performances. And the ideas for his dances always start with the music, he explained during a recent phone interview from his studio in Brooklyn, New York. "That's what I always do," Morris said. "I only work from music." That might explain why the Mark Morris Dance Group regularly performs almost every year at Tanglewood, the summer home of the Boston Symphony Orchestra in the Berkshires. This month, the Mark Morris Dance Group will return to Tanglewood,  where Morris will personally conduct one work and his dancers will perform two pieces, including a world premiere that was specially commissioned by Tanglewood in celebration of the Tanglewood Music Center's 75th anniversary this summer, Morris said.