OK, this took me forever to figure out and googling missed it somehow, there is no GUI solution to this as far as I know.
If you have multiple sound devices and you are using the usb sound card and the volume is too low.
Type “alsamixer” at the terminal.
Then hit F6 and select the usb sound device.
Adjust sound to full max and exit the alsamixer.
Your sound is now fixed and can be controlled from the GUI things as normal.
I need to clarify, the openSUSE KDE GUI mixers and audio hardware setup have the options for usb volume as well. The problem is that the volume as adjusted from the gui only goes up to the maximum that is set with the terminal program Alsamixer.
By default the max volume for the usb hardware is set to 50%. So It looks like you are getting the full volume out of your hardware but you are really only using up to 50% until you adjust it with alsamixer.
I am not sure why this would be different in xfce4 but apparently it is.