Back in Santa Cruz, I marveled at Ukulele Dick and Oliver Brown, maestros of ukulele songsmithing and quirk. But sometimes a song is not just a song. Sometimes it’s an extended, operatic adventure, more like an urban folk ballad, played by San Francisco musician Annie Bacon. Bacon’s Folk Opera debuts tonight at the Noe Valley Music Series and then travels around the country. She composed it entirely on ukulele, with parts for upright bass, fiddle, trumpet, and multiple voices. The voices will take you to another world, one that just happens to be this world.