@gekamon it's not any of that... but thanks for trying.
It's an Opera bug.

Picture-in-picture not working (blank screen, sound on)

I know Opera never care about fixing things... but. Let's try.

Doesn't work anymore. Fix it.

Just Stop. You still don't have a "recovery previous windows" button like moz'ila so Stop This. Everytime I have to re do everything and remember my tabs again for some update I didn't ask. Do better.

@gekamon it's not any of that... but thanks for trying.
It's an Opera bug.

@leocg It's not, actually. I used to do that while using on w7 but after I change to w10 and installed Opera again, this happens.
It's supose to keep playing the audio if you close the whatsapp side bar to check other tabs. It's annoying to have to keep sidebar opened to listen.

Ps.: if you don't use sidebar WhatsWeb and use it on a regular tab this bug won1t pause the audios.