GREAT NEWS - AUDIO PROBLEM RESOLVED! A few minutes ago I opened a Microsoft Edge window to find what version I was running.  I went to "Settings and More, then to "Help and Feedback", and then "About Microsoft Edge".  I immediately got a message that Edge was automatically updating.  I am now running Version 89.0774.57 (Official Build) (64-bit).  I quickly tested the audio response and there is no longer a problem with a loss of volume and frequency fidelity after 12 to 13 seconds of play.  Thanks to everyone who contributed to this fix.
For over a year I have had an audio issue with my Dell XPS 8900 desktop, running Windows 10 and Dell Realtek and Audio MAXX Audio Pro.  The problem only occurs when listening to voice/music sourced from the MS Edge browser.  There is no issue when listening to music files stored on this desktop, or playing a CD using windows Media Player.  There is also no issue when listening to speech or music on URLs accessed through the Internet Explorer browser. However, when I attempt to listen to audio from a URL site on Edge I get full fidelity for approximately 12 to 13 seconds then the audio volume decreases about 30 percent and all bass and treble frequencies disappear. But, I only need to adjust the computer's volume control slide up or down to once more have full frequency fidelity for another 13 seconds.  Also, if I close the volume control window prior to the end of the 13 seconds the audio will immediately return to the low volume and toneless output.  This occurs if you use the volume control located under "Settings" or  "Taskbar". NOTE:  If you do not access a music or speech URL before 12 to 13 seconds elapse upon opening a Edge window you will not think anything is wrong.  You will hear music or speech but it will be bland in tone.  Only after adjusting the volume slide control you will hear full fidelity for about 13 seconds.  I cannot find where the issue is.  All audio drivers are up to date and I have run the built-in self-test which finds nothing wrong.  I need your help and suggestions.  (I have reported this issue to the MS Edge Team back in February.)  
XPS 8900, MS Edge browser audio quality