The mixer uses the "multimedia-volume-control" icon which is part of the icon naming specification. Unfortunately, it is missing in xfce4-icon-theme. However, there is an icon "xfce-sound". Maybe we could link those two?
It is already available inside the xfce4-icon-theme-compliant branch. Which is an attempt to work towards an icon-naming-spec compliant theme. I could use some help moving the icons around to that branch though ^_^.
I'v added the mixer to the panel, but it has no icon and no volume bar. Then i added multimedia-volume-control.pgn from xfce4-icon-theme-compliant branch to /usr/share/icons/Rodent/48x48/apps/, but nothing changed.
Ok, my problem solved. My icon-theme-hicolor package was very old, and had no status directory in the theme, so xfce4-mixer icons was never find.
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