How may I adjust the volume on the 24" Cinema Display while line out is con

I have a Mac mini connected to a 24" LED Cinema display. I also have the same Mac mini connected to an Amateur Radio (Ham radio) system via the line in and line out jacks on the back of the Mini.

All works well in that I can direct audio from various programs to whichever output device I choose but I can not select the Display Audio as the default audio in the System Preferences "Sound" panel.

While there is a switch in the line out jack this switch only tells OS X that a plug is in the jack. It DOES NOT electrically alter the route of the audio as, repeating myself, both the line out and the DIsplay audio can be used at the same time.

It seems at though that the 24" Display Audio device is the only device that can not be chosen as the default device when a plug is in the line out jack. Other USB audio devices can be chosen as default, and thereby adjusted for the desired output level, when a plug is in the line out jack but Display Audio can not be chosen.

Is anyone aware of a software "switch" that will correct this issue?

When booted into WIndows 7, via Boot Camp, this problem does not exist. The display audio and the line out levels may be used and adjusted simultaneously. This again verifies that is is a software switch and not a hardware switch issue.

Any help with this problem would certainly be appreciated.

Mike