XPS 8500 Sound not working

I have a Dell XPS 8500, that has working sound up until a few months ago (can't pinpoint when it stopped working).  It may be related to upgrading from Windows 8 to 8.1, but I can't be sure.  All devices appear to be working correctly (looking at device manager and playback devices) and nothing is muted.  I cannot get sound out of the rear panel or the top headphones connection.  I have tried 2 different sets of speakers, to make sure that was not the problem.  I have also update the Realtek device driver to the latest version available from Dell. 

Also note - I did try to launch the Dell Audio App (as was referenced in other posts, but that app did not come up).  I have run through other diagnostic procedures posted in this forum and elsewhere and I have found nothing wrong.

Any advice would be appreciated.