Currently xfce4-meta defaults to install xfce4-mixer. xfce4-mixer has been abandoned and is scheduled for removal from [Gentoo packages|http://packages.gentoo.org/packages/xfce-extra/xfce4-mixer].  Package has build issues with the latest XFCE packages and won't be fixed.  Currently it can be excluded with -mixer USE flag but it should just be dropped completely.