I don't think this would affect the panel, but under tab settings there is an option for tab muting. You might check and see what that is set to.

@Gwen-Dragon The Mime type is shown as video/mp4; codecs="avc1.4d401e" Is there any way I could tell if there is a codec issue? I've had problems with alert sounds from webpages as well though so would that rule out a codec issue? Its working today which is strange.

I am not sure if I understood your post correctly. The music started playing after a reboot even though vivaldi was not started again? Or did vivaldi was still open or does it autostart?