When folklorist Jim Leary was growing up in Rice Lake, Wis., in the 1950s and ’60s, old-time ethnic music was everywhere. You could dial up the local radio station, WJMC, and hear live broadcasts of Scandinavian music by the Eric Berg Band. The nearby ski lodge was a venue for Slovenian accordion music. Polka star Whoopee John was a frequent visitor from his home base a few hours away in New Ulm, Minn.