As a matter of fact I unchecked it for all my users. These privileges are very confusing then. I guess their proper interpretation is that if:
- checked: the user locks the audio device as long as he wants, no sound switching upon user switching,
- unchecked: the user which currently uses the display has access to the audio device, sound switching takes place upon user switching.
The problem is more general and it regards permissions to access certain devices (and files in /dev directory). Historically if a user needs access to particular device he is assigned to a group which group-owns the device file. It has worked for ages in the unix world i.e. access to an audio device, cdrom, cd burner etc.
This group based approach has several well known limitations. Now it seems that such an access is controlled by consolekit/
See https://bugs.launchpad.net/ubuntu/+source/pulseaudio/+bug/433654 for more details.
And some discussion here: http://ubuntuforums.org/showthread.php?t=1420051
Mind that the sound switching worked fine in previous Ubuntu release (Jaunty).