Comment 1 for bug 576235

the sound problem can be solved by adding your login to the audio group :

sudo adduser your_login audio

I didn't havfe the issue by installing ubuntu from the install DVD, but as soon as I install some updates, I'm affected as well (I reinstalled my computer several times to clarify the symptons)

I think it has something to do with permissions, both the audio & the shutdown problem. Another symptom I see, is that all options are grayed out in "System - Administration - Login Screen".