MacBook Pro and external iSight

At the office, I use my MacBook Pro closed with an external screen and an iSight. Until recently, it worked rather well but now, I have a sound problem. Specifically, in the iChat settings, the Sound item does not show my iSight microphone as it used to. In the video settings, I can choose between the MacBook iSight or the external iSight but this is not the case for the sound item as it used to be.
I cannot date the change with certainty as I don't use it everyday but the most likely change in my configuration is the latest round of updates and security patches over the weekend.

Any idea ?