System Monitor shows cpu at 100% when idle (no apps running), mixer_applet2 is culprit. Reloading applet fixes problem, for a while. The only app that was running before I noticed this was Evolution, Mozilla (flash)?
Looks like a duplicate of bug #418571. I stopped seeing this recently - it might be because I right-clicked on the volume control applet, selected "Preferences", then chose a sensible device (previously it had been associated with a microphone device). This is just a wild guess, though.
I boot on each use, and have not seen it either (in two use cycles), may have been fixed? Mine HAS BEEN set on Alsa Mixer (if that helps).
*** This bug has been marked as a duplicate of bug 418571 ***