after emerging xfce4-meta4.12-r1 I've found no mixer plugin.
I seem to remember it should be installed without any special USE flag.
I searched around and ended on Gentooo Xxce4 page, where it is mentioned as xfce4-mixer package.
After trying "emerge xfce4-mixer" I've go an error about non-existing package.
xfce4-mixer was removed in bug Bug 628424 since it is an abandoned project and had issues with current required libraries.
A suitable replacement is media-sound/volumeicon