ffmpeg -i "S03E05.mkv" -map 0:a:1 -c copy "commentary.aac" (soft subs only):
PSA / Guide: How to properly remux or encode Rick and Morty S03E05 (“The Whirly Dirly Conspiracy”) with ffmpeg rick and morty s03e05 ffmpeg
This works for any episode – just change the filename. ffmpeg -i "S03E05