Posing more riddles than the average sphinx, with its decipherable answers pointing somewhere dark, Song Cycle was anything but passive. I had already witnessed hippie bands playing with their backs to the hall, so the thought of late '60s musicians being interested in their audience struck me as a concept bordering on revolutionary.   The debut album from songwriter and pianist Van Dyke Parks, Song Cycle first appeared in 1968 on Warner Brothers Records. Its twelve songs led listeners through Joycean wordplay and sound collages to reveal messages of dissent and personal loss, at odds with Parks' buoyant, riotously eclectic music. Monumentally ambitious and equally expensive, Song Cycle resembled a film - possibly Citizen Kane - more than the pop music of its day; like Kane, Parks' masterwork was adored by critics yet all but ignored by paying customers. In his efforts to plumb the mysteries of this quixotic record and its subsequent fate, Richard Henderson interviews several of the key figures involved with Song Cycle, notably Parks himself and producer Lenny Waronker.