Okay, guys! I found this topic when I have the same problem, and that's the solution, that helped me and with chance 99% will help you too!
Problem is that we need to reinstall libffmpeg library for Opera. Users that have downloaded Opera from snap install opera doesn't have our problems.
So, the first solutions is to reinstall Opera through command bellow:
snap install opera
If not or you don't wanna use snap, just make this steps:
We need package "opera-ffmpeg-codecs" from Arch Linux's repo.
Archive file name will be another. Don't just copy paste this, take an attention to the commands)