Couldn't you just download subtitles for old movies and train using those?
https://asciinema.org/a/93ihv5er83mrpihh9i5m3s9o5
Congress has video for all of its sessions and it is transcribed. So does the Supreme Court (though not timestamped).
This is enforced by the FCC [0], but as more and more "internet" content gets consumed I imagine the same regulations will eventually come, at which point you've got a fantastic training set.