Folk group the Hillside Singers is no doubt unique in the annals of popular music -- their very existence depended on the success of a television commercial. The roots of the idea lie in the offices of advertising agency McCann Erickson, who wished to hire extant folk combo the New Seekers to perform "I'd Like to Give the World a Coke," a jingle the company had written for Coca-Cola. Scheduling problems forced the New Seekers to decline the offer, however, so McCann Erickson contacted producer Al Ham, who agreed to assemble a group of session vocalists; Ham ended up hiring his...