Traditional styles grow differently depending on who cultivates them and where. For two decades, Old Crow Medicine Show has helped make old-timey American music vital for a generation raised on Nirvana and hip hop, by abandoning the rules that sometimes stifle folk revivalism and growing in whatever way appealed to them. From covering an entire Bob Dylan album to writing one of country music’s 21st century classics, “Wagon Wheel,” Old Crow has consistently surprised and challenged its audience in the most joyful ways possible. Ketch Secor, a founding member and now the main voice in Old Crow, joins Ann Powers in Nashville to talk about the group’s history and play songs from their latest, the band’s sixth studio album, Volunteer.