By John S. McClenahen Steel drums, a mainstay of Caribbean music, may also be an economic sounding board. And what William B. Sparks Jr. perceives is a rising demand for steel drums, plastic containers and corrugated boxes in North America. Sparks is COO of Delaware, Ohio-based Greif Bros. Corp., a $1.5 billion producer of industrial containers. Explaining that demand for industrial shipping containers closely tracks -- and sometimes previews -- economic performance, Sparks says his company's numbers indicate that in North America "we are starting -- albeit slowly -- to come back" from recession. "It's a very steady but mild recovery that we are seeing."