introduce myself

Hello Engelbert, and welcome.

I am not exactly familiar with the GMC 320 but I have done a little digging. Looking at the manual it states that the USB is for data communication, but doesn't state what kind of data. I would assume the data is for the GMC software only. You should be able to use the audio without issue and after reading an old thread you may even be able to program the GMC 320 to transmit to Radmon via WiFi if desired. The outcome of that wasn't confirmed though, so may work, or not.

This is from the manual and below are two links to older threads. One on WiFi and one on using audio. They may help.

Cheers
Simon

Data Port

This model has a 3.5mm analog data output port. It uses standard 3.5mm audio stereo plug, This port is not for the earphone. The audio signal is for third party device and software. The signal can be connected to other devices or computer microphone port directly.

USB Port

The USB port is the standard mini-USB port. It is used for data communication, external power and the battery charger connection. 

http://www.mw0uzo.co.uk/index.php/forum/howtos-and-faqs/681-gmc-320-v5-wifi
http://www.prscardiff.co.uk/index.php/forum/test-forum/477-gmc-320-cpm-in-software-seems-not-to-match-unit