No sound from line out port: MacPro 2012

My MacPro recently lost sound output from the 3.5mm jack at back of the machine. There IS sound from internal speakers, but not from known working external speakers or headphones.

I have checked sound prefs – The mute box is UNchecked.

I have tried all three output options with no change. I have restarted the machine with no change. Nothing makes any sense other than something was damaged while the machine was sitting still right where it has always lived. 

All I can surmise is that some part of the line out system failed internally, and yes, it's an old machine so I suppose that's to be expected.

Am I missing something obvious, or is this output option expired?