This is a rare problem that could indicate that you have a third-party webcam driver that is incompatible with Office Mix. To address the problem, you can try to use a generic Microsoft driver by taking the following steps:
Go to Start and search for "Device Manager".
Launch the Device Manger and scroll down to “Imaging Devices”.
Expand the list of Imaging Devices and select your webcam from the list.
Right click your webcam and click the “Properties” menu item.
In the Properties dialog, click the “Driver” tab.
Click the “Update Driver…” button.
Click the option for “Browse my computer for driver software”.
Next, click the option for “Let me pick from a list of device drivers on my computer”.
Uncheck the box for “Show compatible hardware”.
In the list of Manufacturers, select “Microsoft”.
For the specific Model, select “USB Video Device”.
Click the “Next” button.