Out of interest, I tried to work out how much it would cost. A couple of companies offering subtitling services offer basic subtitling for $1/min, and closed captioning (which I think you need for accessibility?) at $5/min. Assuming 1 hour lectures, that's either $1.2 million or $6 million, though you might be able to negotiate a better rate for a big job.

CC includes description of relevant non-verbal sounds, but it's not clear to me why the price difference is so big.