If Paddy's advice isn't successful, do this:
Quit iChat
Go to the main (not user) /Library/Preferences and move the following to the desktop:
com.apple.DeviceSettings.plist
and com.apple.audio.SystemSettings.plist (you will then have to go the Sound pref panel to reset the sliders)
Open iChat
Not fixed? Try this:
Quit iChat
Go to Users/your account/Library/Preferences and move the following to the desktop:
com.apple.soundpref.plist
and com.apple.systemsound.plist
Open iChat
If that doesn't work:
Quit iChat
Open Activity Monitor
Select show All processes
Find coreaudiod
Click on Quit process icon in upper part of window
Restart iChat
One of these should do the trick. Some other ideas that worked for others (this is a long standing problem with iChat) are opening and closing Garageband (it resets some audio settings) and opening Utilities/AudioMidi setup and choosing to set the rate to 44100.0 Hz.
Welcome to TS!