Verify that your Mac's speakers are turned on and volume is set appropriately.
You can use the suggestions here to troubleshoot your external iSight:
Use your Mac's System Preferences... > Sound > Input menu command to be sure your external iSight's mic is selected and that its level is set appropriately. If all is well, you should be able to see the blue "VU" indicator lights flicker there when you speak normally.
If your problem remains, test with more than one Apple application in a more than one user account. Don't use a "Guest" account for testing. If your Mac only has one account, create a new user account for testing.
Be sure you disconnect all other Firewire items and restart Mac before testing. If the problem remains, try a different cable with our Mac. If necessary, testing on another Mac can also help find the source of the trouble.