Question Sound not working - Dell OptiPlex 990

Sep 24, 2019
I have an OptiPlex 990. My sound has stopped working. Here are some interesting facts.

I have run tests and there are no apparent hardware errors.
When I look at the Manage Sound Devices page, the speakers show Disabled (and greyed out) so there is no way to change it.
I don't get sound from any sound from any web page - tried both Firefox and MS Edge.
I don't get any sound when playing an audio CD.
If I look at the sound monitor - I can see the volume bar moving.
I have plugged the speakers in to both the back jack and the front jack - no sound.
I have plugged a headset into both the front jack and the back jack - no sound.
I plugged a USB headset into the front - SOUND!

Any help is appreciated.