Modulation with PC NG

I recently purchased a new Flex Radio 6400. I am having difficulty getting my transmit audio from my PC to properly modulate the radio. I am using a mic-headphone which is plugged into the audio out and mic input of the PC. The problem is that in order to get the radio to modulate, like the local microphone does, I need to add 30db of boost in my PC mic recording settings. When I do add the boost the noise is so high that RF is transmitted. I also need to bring the mic gain to 100% on the radio. This is only a problem when using the PC on our LAN side.  The mic headphone works well on skype and other recording devices. Does DAX need to be employed here?