Get more sounds and loops for GarageBand on Mac
After you install GarageBand on your computer, additional content — including patches, drummers and Apple Loops — may be available.
Some additional content may be needed to perform certain tasks. In these instances, a dialogue appears, informing you that additional content is needed and asking if you want to download it.
In case the Sound Library is altered or stops working, you can reinstall the patches, loops and other content in the Sound Library to its correct location.
Some additional content can also be downloaded directly in the Library and the Loop Browser.
Download additional sounds and loops
In GarageBand on Mac, choose GarageBand > Sound Library > Download All Available Sounds.
In the dialogue, click Get.
Download additional content in the Library or the Loop Browser
Items in the Library and the Loop Browser that require additional content are indicated by a Download button 
.
In GarageBand on Mac, click the Download button
next to the item name.
Reinstall the Sound Library
In GarageBand on Mac, choose GarageBand > Sound Library > Reinstall Sound Library.
In the dialogue, click Reinstall.