How To: Set or adjust your built-in microphone on your MacBook
Select Audio Input on your Macbook
Click the Apple icon on the menu bar, and then click "System Preferences."
Click "Sound," then the "Input" tab.
If "Line-In" is selected, the internal microphone won't work. You can use an external microphone if you connect it to the MacBook's audio input port. Otherwise, Select "Built-In"
Mute the Microphone
Click the Apple button and click "System Preferences."
Click on "Sound," then the "Input" tab.
Drag the "Input Volume" slider all the way to the left to mute the microphone. The microphone won't pick up any audio when muted.