Comment 5 for bug 373786

It says the bug was fixed in the package xfce4-mixer - 1:4.6.1-2ubuntu1, which is the version of the package in upcoming Xubuntu Karmic as seen in http://packages.ubuntu.com/search?keywords=xfce4-mixer&searchon=names&suite=all&section=all

You can already download and try a testing version of Xubuntu, see https://wiki.ubuntu.com/Xubuntu/Testing/TestingInfo