I removed my volume icon from the tray long time ago since I don't use sound at this machine.
Decided to look what's going there with this configuration.
I don't have it included in panel but I have it in /home/<me>/.config/autostart
When I commented outNotShowIn=LXDE;
line in the file volumeicon.desktop
refreshed the panel
(not sure about refresh but logout/login did it) it "appeared" invisible (something black on black) but is active, here I hovered the mouse over it:
After that I opened the Preferences by right-click on the icon and changed the icon theme from "Default" (Black Gnome I suppose) to White Gnome and now it is seen as a white loudspeaker on the black panel background.