There's the likely problem..... having the USB mic plugged in.
I'm pretty sure MC6 can only deal with one sound card at a time. It sees the USB mic as a sound card (which it is) and you're trying to also use the factory sound card at the same time.
This is the reason most of us do not use the factory card, but choose instead to use a dedicated USB audio interface. That way, there is only one card and one timing signal to worry with. We use the interface for input and output and do not use the factory card for anything with recording.
Apparently, MC6 is using the soundcard in the USB mic to process the signals. That's OK... kind of. A problem might occur during play back because I doubt that you are using the USB mic sound card for playback.....likely you play back on the factory card. Since the timing is not going to be synced between the two, there may be playback sync issues. Then again, maybe not.
The solution is of course to purchase a dedicated sound card/interface to handle all input and output for MC6. You set it up one time, and from that point on it comes up running flawlessly. There is a huge difference between a USB mic (not the best solution by far) and a dedicated USB interface such as something made by Focusrite, M-Audio, Presonus, Roland, etc..... a huge difference.
However, if you're short on money, and what you have works, even though it's a PITA, use it while you save for a better interface.