I use the mixer panel plugin on Gentoo, xfce version 4.6.1.
My problem is that the loudspeaker volume icons are missing. I tried to use strace to find where it is looking for but failed badly because it seems to be only using icon cache files.

Gentoo installs the icon files into 
> /usr/share/xfce4-mixer/icons/hicolor/scalable/status/xfce4-mixer-*
(plus the 16x16, 24x24 and 48x48 variants), so they are available - just the plugin does not find them.

A thread on the gentoo forums [1] brought the solution: Symlinking 
/usr/share/icons/hicolor to ~/.icons - this makes the icons show up for me, but it seems very hackish and just circumvents a bug somewhere deeper.

[1] http://forums.gentoo.org/viewtopic-p-6006746.html