→ ffprobe -v quiet -show_entries stream=duration,start_time in.mp4
[STREAM]
start_time=0.000000
duration=20.400000
[/STREAM]
[STREAM]
start_time=0.637000
duration=19.641833
[/STREAM]
→ ffmpeg -i in.mp4 -af loudnorm -y out.mp4
...
→ ffprobe -v quiet -show_entries stream=duration,start_time out.mp4
[STREAM]
start_time=0.000000
duration=20.400000
[/STREAM]
[STREAM]
start_time=0.000000
duration=19.658667
[/STREAM]