Hi,
Lets talk in broad terms first.
For a Buddy to hear you the Mic as to be connected and turned On in iChat
Taking that it is going to be an Internal Mic we can presume it is connected.
In iChat it is turned On in the Video menu (It will read Audio if you have no camera) and is the one to the right of the Buddies menu.
In here, Microphone Enabled (Or Audio Chat Enabled in Lion) needs to be ticked.
The Mic then needs to be chosen in System Preferences > Sound > Input AND in iChat as the input/Microphone choice.
In iChat the choice is made in iChat menu > Preferences > Video/Audio Section > Microphone drop down.
In the System Preferences > Sound > Input the volume has to be high enough for you to be heard.
This tends to be a setting that moves the blue bars to about 3/4s of the way across. (At least 1/2 way and not banging in to the right hand end when you talk)
If you are doing things in other apps that use the mic differently or even a different Mic you may have problems if you don't manually change things back for iChat.
So...
Transferring this to the Other end, your Buddy may not have the Mic on.
May have been doing something else Audio-wise and not changed things back.
May have reduced the volume for Input for another App and not reset it.
She may also have issues with the OS "Seeing" the Mic
She should check the System Profiler (Information in Lion) and see if in Hardware > USB the mic shows up
You can access this from the Apple Icon Menu and holding the ALT key down.
If it does not show up here then a SMC/PMU reset may be in order.

9:56 PM Sunday; May 13, 2012
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