Specifically the following YouTube doesn’t come down with Audio. I have NOT upgraded to Sierra yet.
Is it something to do with the video upload itself?
Sorry for the delay.
I was not able to reproduce this problem from here, but I can see that YouTube clip uses the OGG Vorbis audio codec which is uncommon for YouTube clips. The next time this occurs, can you send us the log file? You can find it by going to the Preferences and clicking the Find Log File button.