Option 1: No more in use -> Doesn't fit the context; the passage indicates folk music is still practiced by villagers.
Option 2: No more to talk about -> Contradicts the passage; the author is presenting findings, showing there's much to discuss.
Option 3: Nothing much is known -> Fits perfectly; the musical aspect received little attention from scholars, leaving it unexplored and poorly understood.
Option 4: A waste of time -> Incorrect; the author's research and presentation show the subject is valuable and worth studying.
Hence, Option 3: Nothing much is known -> The phrase "closed book" is an idiom meaning something that is mysterious or not understood. The passage clearly states that scholars neglected the musical aspect of folk songs, and "hence, the subject remained a closed book" - meaning it remained unknown or unstudied until the author began primary research. -> correct