The short answer
Usually, after a check. Might hold something you care about. Look inside before removing it.
What is it?
Instruments, loops and lesson content GarageBand downloads, stored in /Library/Application Support/GarageBand. The basic set is small, but downloading all available sounds can add several GB.
Instruments and lessons downloaded by GarageBand. Downloadable again from the app.
What happens if you delete it?
All of it can be downloaded again from GarageBand › Sound Library. Projects that use a removed instrument ask to download it again.
How to remove it properly
If System Settings › General › Storage shows a Music Creation entry, remove GarageBand content there. Otherwise quit GarageBand and move the folder to the Trash in Finder (Go › Go to Folder…), which needs an administrator password; restore it later with GarageBand › Sound Library › Reinstall Sound Library.
From the app itself: GarageBand > Sound Library: remove or relocate content.
Before you delete it
If you also use Logic Pro, some content is shared: manage it from Logic’s Sound Library Manager instead.
How MoreRoom helps
MoreRoom recognises this item and labels it “Review first”: it shows what’s inside, its size and last use, so you can decide. Nothing is removed without your say, and everything goes to the Trash first, with Undo.
Frequently asked questions
Can I delete GarageBand’s sound library on Mac?
Usually, after a check. Might hold something you care about. Look inside before removing it. All of it can be downloaded again from GarageBand › Sound Library. Projects that use a removed instrument ask to download it again.
Will GarageBand sound library come back after I delete it?
Yes. All of it can be downloaded again from GarageBand › Sound Library. Projects that use a removed instrument ask to download it again.
How do I remove GarageBand sound library properly?
If System Settings › General › Storage shows a Music Creation entry, remove GarageBand content there. Otherwise quit GarageBand and move the folder to the Trash in Finder (Go › Go to Folder…), which needs an administrator password; restore it later with GarageBand › Sound Library › Reinstall Sound Library.