Clearly, you know more than the average user. You've given a complete problem description & you apparently know all about ffmpeg. Such a welcome deviation from the norm around here.
I went to your site & found the separate audio & video to download with VDH. They were way down at the bottom of the VDH menu, which I had to scroll down several screenloads to find. This shows the 2 VDH downloads in progress. It also shows the HLS manifest. It turns out the manifest is the only manifest. It consists of just a single Program consisting of one video Stream & one audio Stream. There aren't multiple resolutions on offer here. The Network Monitor shows the manifest, which I suppose is by default the master manifest, and the 2 stream manifests, one for video, one for audio, which is easy to guess from their naming convention. This is rather an unusual site. Normally, I filter on .m3u8 with the leading period. But I had to remove the leading period to get these manifests to appear. It turns out they don't name their files with .m3u8 as a file extension. At least they included m3u8 somewhere within their file names. I would have hated to have had to trawl through the entire Network Monitor listing to dig these needles out of the haystack.
Sadly, the VDH downloads failed after several minutes.
No worries. You just have to turn on this setting.
Reloading the page & trying again did work. Here's those downloads in progress. Note they are now M2TS files instead of MP4s.
The audio download completed in about 3 minutes. The video took 13 minutes. Sadly, these also both failed.
I have said before that I believe that HLS as M2TS setting should not even exist. VDH should figure out when it's necessary & simply do the download. This is rather an unusual case in that even that setting didn't make this work. Neither file I downloaded would play in VLC. You should note that VLC plays M2TS files. There is no need to convert them to some other format. In addition, had these 2 downloads worked, you could have played the 2 files synchronously in VLC without merging them. If you wanted to merge them, you could have used the VDH tool for that purpose:
But like I say, VLC is perfectly happy playing 2 files synchronously, one with only a video track, the other with only an audio track. You can also merge them with ffmpeg, but you seem to know quite well how to do that.
Meanwhile, I downloaded the same video using ffmpeg. I used the URL of the first manifest as the -i for ffmpeg It just found the video & audio tracks in the manifest & downloaded a unified MP4. There was no need to run ffmpeg twice, once for the video & once for the audio. There was also no need to use -map parameters on the ffmpeg invocation since there was no ambiguity in the manifest. For some reason, my ffmpeg download ran at only about 1 million bytes per second. My VDH downloads ran at 3-4 million bytes per second. I can't explain the difference. Web sites are just picky sometimes, I guess. The ffmpeg log showed it was downloading at a speed factor of only about 1.3, meaning about one third faster than regular speed playback. That means our 55 minute video would download in about 37 minutes, disappointingly slow, and not much better than sitting & watching it. Due to the usual fluctuations in download speed, the ffmpeg download actually completed in a little over 41 minutes. It played fine in VLC with audio:
I have to say you would have to be a committed viewer to sit & watch this all the way through. Either you were one of those graduates or you're a member of the Leeds alumni or you're the proud parent of one of those graduates. I am none of those so I just sampled it at 1-minute intervals & it seemed to be playing just fine with audio right to the end. The file was about 2.18G & it had these properties:
That is actually decent quality video at about 5500kbps for the video bit rates, & decent quality audio at 48kHz audio sampling rate & 189kbps audio bit rate. Considerably better that what you typically get from YouTube & the various porn sites other users like to post about here.
The fact that ffmpeg had no problem with this says to me that VDH should have handled this one without a hitch, and especially without resorting to HLS as M2TS, which didn't work anyway. I don't even see why VDH recognized this as separate video & audio. There aren't even multiple resolutions to sift through. This one is quite baffling. At least we have ffmpeg as our fallback.