Plug in a mic or an electric guitar
Every Mac comes with a built-in microphone, and most Macbook and Desktop Macs also include a “Line In” audio input. This Line in port looks identical to a headphone jack, but instead of a headphone logo, it will have a circle with two inward facing triangles.
The port can be used to plug in an external microphone, audio device or recording equipment. Just plugging a device into the port doesn’t make it active by default. You will have to choose Line In instead of the Internal Microphone in System Preferences.
Open System Preferences by clicking its icon in the Dock or Applications folder.
Under the Hardware settings, click on the Sound icon.
The Sound preferences window will open. There are three tabs located at the top of the window, Sound Effects, Output and Input; select Input.
A list of connected audio devices will appear in the window. If any USB or Firewire audio devices are connected to your Mac, they will appear in this list. Click Line In to set the input device.
Below the device window, there is an Input Volume slider. To adjust the gain on the Line In input, drag the slider left or right to lower or raise the volume.