flac -t proves the stream decodes; it does not prove the stream carries anything. A segment whose entire system channel is digital silence passes flac -t with ok and exit 0, which is how a build that recorded no system audio at all reached owners and stayed there for nine days. Drive both legs during the checkpoint window and measure each channel separately. Verified against real segments: the check exits 1 on a pre-fix capture whose system channel is all zeros, and 0 on a post-fix capture carrying signal.