Lesson not to trust Dell update assist.  If still no fix, back up data then wipe disk to do a fresh clean install.  Do not bother with Dell driver as Win10 will automatically take care of all compatible drivers for this pc.  Been there done it before.
I ran a Dell Update Assist and it somehow uninstalled the default onboard sound .. Sound Output now shows   "No output devices found"   but I have the default headphone in use before this.   I think it is realtek , I tried installing that driver but it did not help.   This is an old but very useful computer, but I need to restore the sound, I don't have a system backup/snapshot to restore it.      
Help Please Audio uninstalled Studio XPS 435T / 9000