I have noticed that xfce4-mixer-plugin will randomly start saturating my laptop's CPU. (2.8GHZ Celeron) The problem is so bad that it pushes the laptop to about 150 degrees ferenheit. I have found no cause for this and i have been unable to reproduce it happening, other than it sometimes happens after waking from hibernation. However, this is not the only time it happens. I have found that removing the plugin from the panel and replacing it solves the problem temporarily.
This is probably a dupe of 5183, which was actually a gstreamer bug that has since been fixed there. *** This bug has been marked as a duplicate of bug 5183 ***