I have a different problem (also using Petra XFCE) but sounds like it might stem from a similar overall flaw. As near as I can tell, switching AWAY from any sound output device is permanent until the next reboot.
- After I start up and log in, the default output is the builtin speakers on my laptop. Audio works fine on the speakers.
- After I switch to the headphone output, audio works fine on the headphone jack
- If I then switch back to the speakers, speaker output doesn't work. The audio control panels shows sound levels in the view meter, but no sound comes out.
- If I then switch back to the headphone, headphone output also doesn't work. As above, the control panel shows sound levels in the view meters.
In short, once I've switched from speaker to headphone and back, there's no way of getting sound from anything until the next reboot.
I have tried simply killing and restarting pulseaudio, to no effect.
I have not yet tried logging out of XFCE and back in again, but stopping short of a reboot.
Does anyone know where pulseaudio writes its log messages, and if there is a way to turn up the verbosity of those messages?