This was a head scratcher. Headphones stopped being detected when plugged into the front panel jack and audio output was only to the internal speakers. Long story short, a checkbox on the MaxxAudioPro utility had become unchecked.
Went thru the usual settings to check listed on many, many web pages and got nowhere. Sound in the control panel, sound settings, audio mixer, ran the audio troubleshooter, updated drivers, etc., all to no effect.
Finally realized the select popup had to come from somewhere and searched for apps that might be controlling the audio output. Found the MaxxAudioPro and under advanced settings there is a checkbox for dialog when devices are connected. It was unchecked. Enabling it restored audio to the headphones.
Original problem on an Optiplex but laptops also use the MaxxAudioPro so it’s not limited to a specific model.
Jerry's blog 