That sounds like an issue with Windows 10 turning off short filenames by default, which has in turn caused problems with Windows Media Player's ActiveX control because it needed short file names to work around other issues.
This slipped off the radar but I'll see if we can get that fixed / worked around. Thanks for the reminder. This was fixed a while ago in Opus 12.
When I renamed the mp3 that I copied to the root of the drive to xxxxxxxxxxxxxxxxxxxxxxxThing.mp3 it also played fine in OD. And then I renamed it to 'Wild Thing.mp3' and it would not play at the root. So, do you still think that is an 8.3 short filename issue? 8.3 standard is not just the length of the filename is it? It also doesn't accept anything except a-z, 0-9, and _, correct?
I have OD 11, so would that also be fixed if you fixed it? Or just OD12?
Wish there was a way that I could fix it..
Windows Media Player's ActiveX component has problems with spaces in names, which short names were used to work around. So it's the space, not the length.
Updates will only be for Opus 12 at this point.
Ok. Thanks for your help.
I went to fix the problem with Windows Media Player and spaces in filenames on new Windows 10 installs (with short names disabled), only to find I already had fix it.
It has been fixed in Opus 12 for a while, in fact (Opus 12.0.5). I remembered the problem, but not fixing it. 
where is the software URL? download link?
The MP3 plugin was for editing MP3 tags in the viewer window. It was included in older versions of Opus.
It's no longer included as that functionality was replaced by the metadata panel.
If you just want to play mp3 files, the ActiveX plugin should handle that via the Windows Media Player preview handler (click to play) or activex control (auto-play).