I have kind of the same problem with the sidebar. The sound on the WhatsApp and Messenger sidebar apps doesn't work. On Telegram the sidebar app doesn't even work, but if I access the Telegram web page through the browser it works perfectly. It all works fine on Twitter and Instagram.
I'm on MacOS 10.14.6, my Opera version is 69.0.3686.77 (the latest). I've had this problem for some months now, about 5 moths or so. I thought it would be solved after some updates, but it's been a long time and I cannot see any solution.
I've already tried restarting my computer; closing and relaunching Opera; logging out and in again on the apps; but the problem keeps occurring.
I hope this can be fixed soon, since this browser is, in my opinion, the best one out there.
Thanks in advance!