Error when launch gvolwheel (application which lets you control the audio volume):
cannot open mixer device
Then launch volti (another application for controlling audio volume), all Ok.
Close volti, and again launch gvolwheel - successful! But after reboot again this same error…
It reads to me that some app has seized the audio device upon boot where gvolwheel can not access the device as another is using it. For some reason volti is able to seize the device away for use. And then when you close volti it properly closes its access to the device, leaving it open for another device to use, which in the case of your efforts is gvolwheel.
Are you using pulseaudio ? I suspect not , but my suspicions could be wrong.