Recording sound from Line In & Mic

Open the Audio MIDI Setup application (in the Utilities folder inside the Applications folder). In the audio devices window you will see a list of all available devices connected to your computer. Click the plus button to add an Aggregate Device. Check the box next to each input device that you want to use.

Now, in Garageband go to the Preferences and click on Audio/MIDI. Select the Aggregate Device for Audio Input. You should now be able to use all the devices you selected with the Audio MIDI Setup application. You will see that each device has been assigned its own inputs which you can select for each track in Garageband.

You may find that you can't individually adjust the volume of your microphone in GarageBand however. If so, select it in under the Aggregate Device in Audio MIDI Setup. You should see volume sliders there that you an adjust. If needed, you can have the Audio MIDI Setup application running at the same time as GarageBand and make adjustments as you go.

Glad to help.