Do you, for any change, use gstreamer 0.10.7 or another version than the one from current dapper? This looks exactly like http://bugzilla.gnome.org/show_bug.cgi?id=344474 which was introduced with gstreamer 0.10.7 and fixed in 0.10.8 -- mixer applet no longer works https://launchpad.net/bugs/49537