Like movies about kids who spell or athletes in wheelchairs, a non-fiction film about old people who sing rock, funk, punk, and blues in a community chorus that has dazzled international audiences on concert tours comes formidably fortified by its ironclad charm factor. To be honest, I settled in skeptically, expecting to be gummed into submission. Two dozen or so amateur singers, based in Northampton, Mass., lend their group's name to Stephen Walker's surprisingly profound documentary.............................