Troubadour Pitches In Despite His Misgivings By JIM FARBER Daily News Feature Writer Rock fans know him as the voice of heartland America. But John Mellencamp has felt out of step with his U.S. concert audience ever since the events of Sept. 11. "In the shows, people would chant, 'U.S.A! U.S.A!' It's frightening to me," he says. "I would go into another song so they couldn't get it going." Click here for rest of article