I am having a problem with my microphone. Plugging in an external microphone does not disable the onboard mic. (1) When I plug an external mic into the jack on my SP, I can hear both mics. (2) The 'microphone' in the Sound control panel says "not plugged in." (3) The Surface onboard mic device icon is active (4) If I disable the SP mic, both internal and external mics turn off. Any ideas?