This is a problem that plagues YouTube. YouTube. It plagues YouTube. VDH is just one
of the victims. Sometimes you hit a video in which the video & audio are in separate
files. You can usually tell in the VDH menu which one is the video without audio & which
one is the audio without video. The video will show a resolution, something like 640x480
or 1920x1080. The audio will not show a resolution. The two entries will look otherwise
similar. You have to download them both then play them synchronously in VLC. How do you
do that? Click this link:
https://groups.google.com/g/video-downloadhelper-q-and-a/c/BzPLK2YyL-s
You'll probably want to bookmark that for easy reference. Do a string search on
"synchronous" in there. That will give you a link to instructions how to do that.
Then there are times when one of the resolutions will include both video & audio and the
other resolutions will not. There is no way to figure this out other than by downloading
the various resolutions until you find the one that works. In such a case, you might
discover that the 1920x1080 variant that you want has only video or only audio. But the
640x480 variant is the highest resolution at which you can get both video & audio.
That's just the way it is. VDH is at the mercy of what's there on the web site.
It would be helpful if you posted at least one URL where you have observed the problem.