XFCE 4.3.99.1 installed on Slackware 11.0.  All user accounts are unable to add the mixer plugin to the panel.  Starting xfce4-mixwer from a terminal results in the following message:
xfce4-mixer: error while loading shared libraries: libxfce4mixer.so.0: cannot open shared object file: No such file or directory

The library file is located at /usr/lib/xfce4/modules/libxfce4mixer.so.0.0.0
but the mixer does not seem to be finding it.  If I create a symlink in /usr/lib/ to libxfce4mixer.so.0 it will load and work. Other panel plugins are working normally.