Are subtitles done by a human or a computer?
Without knowing the definitive answer - I would say both. My wife, although not hard of hearing, likes the subtitles on and sometimes you get descriptions of sounds... examples:
"Ringing telephone" "large swooshing sound" "thunderous applause"
Surely a computer cannot analyse and identify those sounds?
Then there's the background music that is always identified, a human would have to be standing over the computer ensuring all is correct. Although there is software to identify songs like 'TrackID' on mobile phones it often gets it wrong.