Well, not long after I posted the above issue, I solved it. With the help of a post here on a slightly different topic. I went to the Device Manager and clicked "View/show hidden devices." There I saw several drivers under "Miracast," which I've used to connect my XPS 13 to a TV, mostly recently to a Samsung Smart TV. I remembered that this was right around the time my video stopped working. So I uninstalled that Miracast driver (as well as the three others that were Roku drivers) and Voila! My videos now work correctly. Whew!
An odd thing happened about a month ago. Videos on Twitter, Facebook, YouTube & my web pages stopped playing. I use Firefox, so I switched to Chrome and Edge and got similar results. The only difference is that the videos ultimately played on Chrome and Edge, but took a very very long time to load. I can play videos just fine on my XPS 13, as long as they aren't being viewed in a browser. I have tried every solution that I've found on the net, clearing the cache & cookies, temp files, reinstalling the browser, reinstalling the video display driver (Intel Xe Iris) but nothing works. I've scanned my computer with Malwarebytes and Hitman and found no viruses. All my drivers, BIOS, Windows 10, etc. are up to date. Does anyone have a clue? Again this just happened sometime before Christmas. I don't know of any software or plug-ins that I installed that caused this. BTW I am a video producer and edit videos weekly for posting (using Adobe Premiere Pro) and have never had a playback problem on Twitter, etc.  
XPS 13 stopped playing videos on Twitter, FB, etc. but still plays non-browser 
videos