I just hover my mouse cursor over the tab with the audio and I click. There is suddenly a mute button that mutes the tab.
On Chrome, the offending tabs always display the mute button. So I can see them and I don't click on them. Also, I can disable them, so even if I do click on them, they relay information, but they don't interfere with the media on the tab.
The sides are switched, on Chrome they are on the right hand side. On Opera, they appear on the left hand side of the tab. Not that this matters, but I'm just assuming someone went through a lot of trouble to make things on Opera different than what they already get with the Chrome engine.
I just saw now that I can install older versions of Opera from the FTP site. I would prefer using the latest and greatest versions, but if that is what it takes, I recall that there was an Opera version without the featuritis that just did its job. Even the developer versions were working. I'll see what that version was and then just disable updates so I can disable this mute button.