On the transcript, does the red sideline mean the stall warning is active and audible?
The legend is in the top right corner of page 10 of the DFDR document.
The red sideline indicates the stall warning is active and (presumably) heard on the tape. When the stall warning is not active, the C chord is active almost the entire remaining time (stall warning not active) except for a brief interval just after the Captain entered the flight deck when they transitioned the correct FL.
My thanks to HN39 for straightening out my misconception about the signals being interleaved. Stall warning has absolute priority while active.