Following this post: https://askubuntu.com/a/1164973/574240
I was able to get media keys working again by enabling
xfce4-notifyd. Brightness and media keys work.
The problem is, if I unplug headphones even once (from either an external sound card, or internal sound card jack), the ability to use media keys stops. The keys won't change the sound for the headphones. Brightness and other keys still work.
The only resolution is a computer restart to get the notify / volume change working again on the headphones.
Restarting the notify service, nor pulse audio changes this.
18.04 xubunu doesn't seem to have xfce-mixer packages anymore. The new functionality is also fairly frustrating with all of these small bugs with sound.
Any idea how to fix media keys so that even if headphones are re-plugged in, the system will listen for them?