Hi, Jim -
Thanks very much! That fixed me up!
I assume that the Windows 10 Update "automagically" associated Windows Groove Player with the MP3 file extension - without my knowledge or consent.
In Windows Settings, re-associating mp3's with Windows Media Player did the trick.
Anyone else bumping into this - be aware that you need to reboot after making the change. Save your tunes early and often.
And BB developers: - further research turned-up many online statements that "Windows has begun associating mp3's to 'more-modern players', such as Groove-Player." No gripes: I LOVE BB and I don't care what player I need to use... but I scoured my manual front to back, and the file association is never mentioned. A couple sentences circa p.74 in the Manual will fix this easily!

Back to making music, now. Cheers!