What is this microphone image from? It appeared on my friend's MacBook and she can't figure it out.
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This is the Mac OS X built in voice input mechanism.
To turn it off, visit preferences - it's possible to turn it on via a keyboard shortcut which may explain it's random appearance.
Speakable Items, built into OS X and located in the Speech pane of System Preferences, lets you control the computer using your voice instead of the keyboard. And you don’t have to train your Mac to use it. You can use Speakable Items to navigate menus and enter keyboard shortcuts; speak checkbox names, radio button names, list items, and button names; and open, close, control, and switch among applications
This article from Lifehacker explains how to use the feature is you are interested in learning more.
If your friend navigates to the Speech pane of her System Preferences she'll see that Speakable Items is enabled. Speakable Items is a limited form of voice recognition that allows you to issue commands to your Macintosh vocally. The microphone device you see indicates that the system is listening and, if I remember correctly, can also be used to toggle the listening state of the Mac.