If I may help.?
I had similar isue with my new old workstation Dell T7500.
System sounds was working with my onboard card, Media player too.
But Opera (Youtube, Facebook) not!
So, what I did.
In Device manager I opened the Sound controler.
There was except a High definition audio device, more 4 NVIDIA audio devices.
I disabled them from the driver page, and things happen.
My conclusion:
In my case, Opera see as defolt device, first Nvidia outputs.