Sound Card / PC350 Issue

Hey guys. I am having an issue with my sound/sound card/headphones. I have a pair of PC350's as well as a Xonar DX that I just purchased. My issue is that when I flip the switch to disable the microphone on the PC350's, a lot of static goes through as well as a very quiet version of my headphones sound. This has been worrying me a bit. Is this supposed to happen? I will be on skype and everything will be fine but if I flip the mute mic switch on the PC350's, everyone can hear the sound from my computer (although it is quiet). This happens both with and without the Xonar sound card, as well as still happens if I disable all my sound devices except for the Xonar drivers. Any idea? Thanks!