Comment # 5 on bug 1093578 from Patrick Finie

(In reply to Takashi Iwai from comment #4)
> OK, thanks, now I understood better.
> 
> So it appears more likely an issue with XFCE pluse plugin.  Basically the
> audio keyboard control is taken by that application and translated to the
> appropriate action.  Usually this should have been applied to the main
> PulseAudio volume control.
> 
> When you disconnect the USB audio device, it starts working again
> immediately?
> And, you can manage the volume control via the volume bar on the task bar
> for multiple devices, right?

Going to the Inspiron 3850 as an example,  i unplugged the Triton Kunai to take
with me on a trip.  The volume controls on the keyboard still do not function.  

Unlike KDE, most of the audio things for the panel only control one audio
source and to deal with multiple audio sources or streams you have to open
Pulse Audio Control.