Volume control via media keys in xubuntu

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http://askubuntu.com – I noticed that the volume control when I use xubuntu is not really working properly. I can see, that xubuntu is recognizing that it should do something and displays a small image indicating volume up/down or mute. But, it is not affecting the sound I hear for instance while using youtube. When I open my PulseAudio I see in Output Devices Tab two devices: Built in Audio Digital Stereo and Built-in Analog Stereo. The indication for sound volume for the first device actually does react when I press the media keys for volume up/down. But this should be the second device that should be controlle (HowTos)