This discussion has helped me to obtain mp3 support, https://solus-project.com/forums/viewtopic.php?t=1004 ; post of May 20, 2017 there. I have downloaded an archive with libffmpeg.so file from GitHub (https://github.com/iteufel/nwjs-ffmpeg-prebuilt/releases/tag/0.20.3) and copied the .so-file into /usr/lib64/opera/, previously saving the corresponding old file as libffmpeg.so_orig, restarted Opera and it works.
libffmpeg.so from Chromium didn't help for me. FireFox and Chrome did play content, but Opera didn't.
My OS is openSUSE 42.2, 64-bit.