Comment 10 for bug 901338

For choose the mixer device:

pactl list | grep -i description

(Si tu Linux es en español como el mío, debes de escribir "descripción" ;-) )

Grab your device name, and remove all the non-english-alphabetic characteres (like spaces, brackets, accented vowels, etc.). E.g: This device name:

 RS880 HDMI Audio [Radeon HD 4200 Series] Estéreo Digital (HDMI)

becomes:

 RS880HDMIAudioRadeonHD4200SeriesEstreoDigitalHDMI

Let's call it "{STRIPPED_DEVICE_NAME}". And then run:

killall xfce4-volumed
xfconf-query -v -c xfce4-mixer -p /active-card -s Playback{STRIPPED_DEVICE_NAME}PulseAudioMixer
xfce4-volumed

In my case, the internal audio device is called "Audio Interno Estéreo analógico", so set the active card to "PlaybackAudioInternoEstreoanalgicoPulseAudioMixer" did the trick :)

YMMV