Just got it working. I used 0.31.4-linux-x64.zip (found here:
https://github.com/iteufel/nwjs-ffmpeg-prebuilt/releases/ ) and replaced the existing libffmpeg.so file in the folder /usr/lib/x86_64-linux-gnu/opera/
Extract the zip file to a folder, f.e. home/user/
sudo rm /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so
sudo mv /home/user/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so