Radio days had already begun to wane by the '50s and '60s, but in some pockets of the country, loyal listeners continued to tune in even after TV mania had started its sweep. In Echoes & Postcards, a theater piece structured like a vintage radio variety show, Tennessean theater ensemble the Road Company pays tribute to the country music of that era and its influence on the denizens of a small southern Appalachian town. Echoes is a musical collage of scenes, skits, comedy, monologues, and stories woven together with adaptations of melancholy country standards like "The Tennessee Waltz," "Your Cheatin' Heart," and "Love in Vain," as well as original songs written by ensemble members the Brother Boys. The 24-year-old company, which hails from East Johnson, Tenn., and has dedicated itself to creating theater about people, places, and concerns from home, draws on its collective experience to create a parade of characters, from Southern belles to philosophical country store proprietors, evoking a time that exists only in memory now. The show opens Wednesday at 8 p.m. (and runs through May 3) at A Traveling Jewish Theater, 2800 Mariposa (at Florida), S.F. Admission is $18; call 399-1809.