You don't tell us your Mac OS, but my guess is that your MacPro is not using the latest version of Lion.
The fix is to apply the latest Lion Combo Update: http://support.apple.com/kb/DL1524
Although not specifically mentioned in the About the OS X Lion v10.7.4 Update
article, Mac OS X 10.7.4 includes a new FaceTime Version 1.1.1 that allows use
of external iSight's mic.
Users of FaceTime 1.1.1 (and later) can now select external iSight's mic
in the FaceTime > Video command menu as shown here:
In earlier Lion versions, (external) iSight did not appear in the "Microphone" list
of the FaceTime > Video menu.
(If you already use 10.7.4, reapplying the Combo may correct your FaceTime.)
Mac OSX 10.7.4 External iSights