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It is enabled from the system preferences AND in the application that is attempting to use the mic?
 
in iChat it should say "Internal Microphone" in the Audio/Video Tab

in system preferences under sound then input it should also be "Internal Microphone"

When you are in system preferences, does anything show when under the input level when you are speaking into the mic? Also make sure that the input level is raised up a little. (The little blue pentagon)

Also, restart iChat and if that does not work, restart the computer.
 
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