Maybe you don’t even need that, at least for accessibility.
Windows for example now has exactly this feature, which is a speech-to-text-transformer powered by some “AI”.
But, in contrast to the Bing chat, this works (afaik) offline by some FOSS-backend, which I don’t know the name of anymore (maybe someone else will?) You could use that tool for live transcription. That is supposed to work extremely well!
Please correct my if I’m wrong, I don’t use Windows anymore personally, and at work we have a business edition that doesn’t ship this brand new feature yet.
(Side note: as strongly as I hate Windows, this feature is absolutely godsend for hearing-impaired people and should be adopted by every other OS!)
If you want to transcript movies and thereof in bulk by uploading them, I can’t give you any information, sorry.
But I believe there are some sites that give you the “subtitle file” for download freely, which you can add manually for each movie in Plex/ Jellyfin.