I just tested my camera out (also a C920) with this site: http://www.testmycam.com/ and the video worked fine. I used Win7x64 and IE9 with Flash Player 11.6.602.180
Picture loaded up fine. I tested the microphone out using this page: http://www.midomi.com/index.php?action=main.mic_check
Again, no problems recording. Could you try updating your version of Flash Player and give these pages a try?
Thanks Chris for the suggestion. I loaded the new version of Flash Player, went to testmycan.com and it still did not work. I then tried the camera on my old machine and it worked. I did notice that the window that asks you to Allow or Deny Camera & Microphone Access pop up on the screen for my Windows XP machine, but did not on my Window 7 machine. I am not sure if it matters for this pop-up window, but I did have pop-up blockers turned off. Not sure what settings would cause this. My flash privacy settings are set to ask for permission to connect.
Allow the camera on your Flash Player Settings. Right-click on any Flash content, then select Settings.