The volume control icon is missing from the panel. The n/w and battery icons are shown. When I try to edit the Notification Area from the Panel preferences I see the volume is not available as a known app. I need help. Am a newbie :)
The volume control icon is part of the xfce4-indicator-plugin, which can contain various icons. You can accidentally remove the volume control icon, which will close the plugin; when it is running it is listed in system monitor as xfce4-indicator-plugin.
To return to your old panel setup as displayed here: 
, please follow these instructions.
(Only the volume and mail icons are actually part of the indicator plugin; the networking and power-management icons are separate plugins).
1) Just right-click the panel and go to panel preferences, as in the screenshot below:
2) Then, just make sure the appropriate panel is selected, move to the items tab and click the green + in the items tab (not the one at the top of the screen) to add an item.
3) Now, scroll down the list and select the indicator-plugin, click add, and close the menus. Your volume control icon and mail icons will be restored.
This should sort out your problem unless you have uninstalled some xfce4 items or icons.
