Wanted to let you know that I fixed the problem, it appears. Pulse audio pasystray seems to be the issue afaiu. Below are the steps to make the multimedia volume keys work on pinebook pro:
1. Open Keyboard from Settings and remove the volume shortcuts from the applications shortcut tab.
2. Open Session and Startup settings and deselect Pulse audio pasystray to disable from autostarting.
3. Reboot to make sure you don't have pasystray starting on the notification area.
4. Right click on the panel to add "Pulse Audio Plugin" to your panel.
That is it. You may want to reboot once more to ensure all your panel items are in place. Pulse Audio Plugin of XFCE is better suited for daily usage than pasystray imho as it is less cluttered.
