For 46 years, this soulful oddity has been San Francisco's best-kept musical secret FacebookTwitterEmail Jazz piano melodies and San Francisco street recordings bounce around the room like ping pong balls from speaker to speaker.Mariah Tiffany/ Special to SFGATE In the Audium’s pitch-dark concert space, sound is the only color. Since 1975, this theater hidden off Bush Street in Polk Gulch has quietly — and sometimes loudly — tried to redefine how San Francisco thinks about music. The humble performance space feels like a college lecture hall crossed with a mid-century modern rec room. S...