OK,
There are two places to check. Actually two type of places to check.
1) Go to the iChat > Preferences > Alerts section.
Check every potential setting in the top drop down with whether it has a AppleScript setting checked in the box below.
2) Click once on every Buddy (in turn) then hit the Apple key and I together to get the Info card up.
Go to the Alerts tab.
Repeat 1) (This is because you can set the Alerts Individually as well as Globally in 1) )
Once you have found and removed any setting to Use one of the AppleScripts you can then use the iChat > Preferences > Alerts section to open the Scripts Folder and open the Scripts with AppleScript Editor (Just Double Click them) and check them for changes (there is an Auto Accept that is phrased to cope with all types of chats and Screen Sharing invites there is also an Auto decline)
If this fails to work you will have to delete com.apple.ichat.plist
This will mean making your Settings again in all the Preferences as you Lose Alerts settings, Font choice, Font Colour, Bubble colour, whether the Menu Bar icon is showing, Whether Transcripts are Saved and so on. You also lose any Personal or Individually set Alerts.
Outside of the Preferences you will also lose any Saved Status Messages you have Created.
You will not lose your Buddy List or your Buddy pic or those of your Buddies.
(You may lose Animation if set it.)
My guess would be that the choice for
Message Sent has an Applescript check against it and that the AppleScript it is connected to is nothing to do with Sending.
8:27 PM Monday; December 1, 2008