I've noticed that sound becomes unavailable to me when someone else is logged into my machine and playing music (or has facebook open) in the other account. I've had to ask them to unlock their account and turn it off so I can get sound in my own stuff. Even in sound preferences, the hardware itself disappears and output is "dummy sound".

Is there a way to prevent this from happening?

What would be really good is if I could turn down the volume (or mute entirely) all the sounds on all other accounts on a per-user basis from my sound preferences without affecting whatever setting they have - essentially saying whenever user A is logged in, all sounds from user B's account are muted and anything from user C's account is at 50% while I can still have my own at full volume.