That’s strange. Did you get the “YouTube error. Check the log for more information” message when trying to play music videos like this one with 6.8.9 as well? Originally the 6.8.10dev was released to fix that bug. I haven’t read about people who could play videos but don’t get audio.
I think the fault lies in lines 8662–8673.
2021-02-18 13:31:26.095 T:3787423968 DEBUG: CAddonSettings[plugin.video.youtube]: loading setting definitions
2021-02-18 13:31:26.095 T:3787423968 DEBUG: CAddonSettings[plugin.video.youtube]: trying to load setting definitions from old format...
2021-02-18 13:31:26.100 T:3787423968 DEBUG: CAddonSettings[plugin.video.youtube]: loading setting values
2021-02-18 13:31:26.102 T:3787423968 DEBUG: CSettingsManager: requested setting (youtube.api.enable) was not found.
2021-02-18 13:31:26.103 T:3787423968 DEBUG: CAddonSettings[plugin.video.youtube]: failed to find definition for setting youtube.api.enable. Creating a setting on-the-fly...
2021-02-18 13:31:26.103 T:3787423968 DEBUG: CSettingsManager: requested setting (youtube.api.key.switch) was not found.
2021-02-18 13:31:26.103 T:3787423968 DEBUG: CAddonSettings[plugin.video.youtube]: failed to find definition for setting youtube.api.key.switch. Creating a setting on-the-fly...
2021-02-18 13:31:26.103 T:3787423968 DEBUG: CSettingsManager: requested setting (youtube.get.watch_later.playlist.id) was not found.
2021-02-18 13:31:26.103 T:3787423968 DEBUG: CAddonSettings[plugin.video.youtube]: failed to find definition for setting youtube.get.watch_later.playlist.id. Creating a setting on-the-fly...
2021-02-18 13:31:26.103 T:3787423968 DEBUG: CSettingsManager: requested setting (youtube.search.remote.friendly) was not found.
2021-02-18 13:31:26.103 T:3787423968 DEBUG: CAddonSettings[plugin.video.youtube]: failed to find definition for setting youtube.search.remote.friendly. Creating a setting on-the-fly...
2021-02-18 13:31:26.104 T:3787423968 DEBUG: [plugin.video.youtube] HTTPServer: Starting |0.0.0.0:50152|
Looks like the addon can’t find its API key. You did go though the process of creating your own API key and telling it to the addon, right? Were you logged into a YouTube account with it? Make sure the addon still has the API key.
I found one of the lines from your log in this post but no reply to fix the issue. I’m out of ideas except for uninstalling it, removing everything else it touched, and reinstalling. Sorry, mate.