Rather than having only one default icon, the mixer plugin would do well to integrate with the GTK icon theme. There are a few icons on my system (gnome-mixer, gnome-mixer-applet, gnome-settings-sound) that would do the trick, but I don't know if there is any standard to which a theme should adhere.
It already does. I forget what icon name it uses though. Perhaps that may be improved (perhaps by using a list of filenames, the panel has code for that).
I copied the icon gnome-settings-sound from the Lila-Red theme to gnome-audio (which is the one the volume control plugin was using from the Rodent scheme). Removing and readding the plugin caused the panel to crash. As such, I'm going to report that to xfce4-dev@xfce.org.
please try to reproduce it with a backtrace. like this we dont have enough info to find the bug at all. (gdb xfce4-panel, run, bt)
Yes, I forgot about this bug; it's actually fixed in CVS since the last time I looked.