AJAX, Ontario ROB MacARTHUR, a part-time music promoter, has a cardboard box full of nearly 60 CD's at his bar in this town half an hour east of Toronto, sent to him by independent Canadian musicians hoping for a shot at getting their songs on the radio. For many of these artists the odds are slim, admits Mr. MacArthur, who is a guitarist in country bands himself. The music is good, but there isn't enough space on the radio for everyone, said the 43-year-old, who with his long gray hair and ready smile has the look of an aging country...