I found the excellent 123Apps Online Audio Converter, which extracts the audio from any online video. No sign-up needed. I fed it a Web link to a 6Gb MIT conference video. (Sadly that was the only option MIT offered, but not all users have i) superfast Internet, ii) the spare disk space, or iii) a video editor that can handle such a beast of a file without crashing).
The 123Apps service downloaded the video onto its servers speedily (about 5 mins). It then offered me a .MP3 of the audio from the 6Gb video, with conversion to .MP3 taking another minute or so. The .MP3 then downloaded with no hassle, at a comfortable 495Mb containing a day’s conference audio at a good quality.
Similar services appear to place limits on the online video size they can digest, such as 500Mb or 150Mb. Which means it’s useful to know that 123Apps can handle very large video files, and that it works very smoothly.