Easy solution via Terminal (rooted):
$ sudo snap install chromium-ffmpeg
$ sudo cp /snap/chromium-ffmpeg/current/chromium-ffmpeg-104195/chromium-ffmpeg/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so
I chose 104195 version because is the only one workable over Opera version 80.0.4170.72 and Ubuntu 20.04.3 LTS (x86_64; LXQt)