On preview panel sometimes mp3/wav won't be played and icon is shown instead.
Today happened: On dual when selecting a mp3-file on LEFT lister, an icon was shown. Selecting a mp3-file on RIGHT, it was played (both same encoder/quality). Tested a few times, left always showed icon, right always played.
Funny: I switched to browser for writing this, left DO open with selected mp3-file on the left (icon shown) and few minutes later suddenly file begins to play! 
It's not Opus playing music files in the viewer, so whatever the problem is will be down to whatever is playing them. (Presumably Windows Media Player ActiveX, or the WMP Preview Handler, and the codecs/splitters involved.)
But I changed nothing except adding flac to active-x for previewing. And never had problems with previewing mp3/wav.
Coincidence. Adding .FLAC to a list of file extensions would not and could not affect anything to do with .MP3 files.
Playing some FLAC files could conceivably leave the playback components in a state which mean they then didn't work with MP3 files, if there's a bug in those components. It seems unlikely, though, and either way is down to those components rather than Opus. Opus does not play FLAC or MP3 files itself.