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This worked on Ubuntu 20.04.1 with Opera 72
Install the "Chromium Ffmpeg codecs" snap via the "Ubuntu Software" app

Then, on the command line, let's find the libffmpeg.so library that was installed by the chromium snap
$ locate libffmpeg.so
/opt/Postman/Postman-1605981456437-old/libffmpeg.so
/opt/Postman/app/libffmpeg.so
/opt/Postman.bak/app/libffmpeg.so
/snap/chromium/1399/usr/lib/chromium-browser/libffmpeg.so
/snap/chromium/1411/usr/lib/chromium-browser/libffmpeg.so
/usr/lib/slack/libffmpeg.so

We want the latest chromium one. Plug that path into the next step

$ cd /usr/lib/x86_64-linux-gnu/opera; sudo mv libffmpeg.so libffmpeg.so.bak; sudo ln -s /snap/chromium/1411/usr/lib/chromium-browser/libffmpeg.so