OS X Yosemite: Adjust your computer’s sound input settings
If your Mac has separate ports for input and output, the sound input port has a microphone icon. If your Mac has only one sound port, it has a headphones icon () and can be used for both input and output.
You can use your computer’s internal microphone, your display’s microphone (if it has one), or an external microphone you plugged into your computer’s sound port.
Before you start, open the Input pane of Sound preferences (choose Apple menu > System Preferences, click Sound, then click Input).
Choose a sound input device
From the list of input devices, select the one you want to use. All sound inputs available to your Mac are listed. If your display has an internal microphone, it’s listed as “Display Audio.”
Adjust sound input settings
If you’re recording sound through your computer’s sound port, you can adjust the input volume to compensate for the sound source being too loud or quiet.
For example, if you’re recording loud music, lower the input volume so the recorded sound isn’t too loud or distorted. Or if you are recording someone who is speaking softly, increase the input volume so your Mac can better capture the person’s voice.
To capture less background noise when using your computer’s built-in microphone, select “Use ambient noise reduction.”