Here's a partial solution. I reached out to Waves who owns the MaxxAudio Pro application, and they suggested that I uninstall drivers/apps and then reboot before reinstalling them.  So I’ll try it again, uninstalling: …rebooting and then reinstalling from Dell.com/support: …and then Rebooting again with headphone jack inserted – audio plays thru headphones – yippie!!   However, unplugging headphones causes all audio to stop.  No sound thru main laptop speakers or Bluetooth devices. This is a symptom seen before -- which shows the audio is again not working...   Rebooting again (just for fun) with headphones NOT inserted  Audio now plays thru main speakers and/or bluetooth speakers -- but again NOT thru headphones.  Rebooting again with headphone jack inserted – audio plays thru headphones.   SO what's the solution? Reinstall REALTEK driver and Dell Optimizer and MaxxAudio Pro APPs, and then always boot with headphone jack inserted?
Plugging in headphones -- they are not detected. After installing Dell Optimizer and then MaxxAudio Pro app, still no headphone audio.  Both apps are required for headphones to be detected, and you must set the options in both apps to "pop up when detected".  Also Realtek audio driver must be up to date, which it is.  Yes, I hear a bit of static (thru the main speakers) when I plug the headphones in, but headphones are not detected by the OS, so audio is not transferred to the headphones.   I see numerous community postings here for a variety of hardware, so it seems a software/driver problem.  And my efforts working with Dell have all been wasted. Any users found a fix for this?  
Headphones not detected - Windows-10 on Precision 7770