How to play stubborn opus files...
First... Open up mkvtoolnix-gui. Next drag n drop the opus file onto the input tab. Then, check the destination and click on start multiplexing. It's done when progress says 100 percent. The .mka (matroska audio) file contains the opus and foobar2000 will play and/or convert it. Close the mkvtoolnix-gui tab and continue. Vlc media player can also play both files. Matroska containers are lossless and very compatible to a lot of stuff.