Mental note for the next time I take my laptop to a meeting.

Undocking the laptop will gray out the headphone. This makes sense because the headphones are plugged into the dock jack. Putting it back in the dock will not restore it.

You will need to go into Sounds > Playback.

1. Enable the Headphone option. (NOT the same as “Speakers / Headphones” as that plays through the speakers on the laptop.)
2. Set Headphone as the Default Device. This changes the applications to use it.
3. Make sure the Speakers (Plantronics) are set to Default Communications Device.

Probably USB headphones automatically restore. At least, the USB ones I have for phone calls did. (They suck for wearing a long time for listening to music.)