I found no way to undo the sorting inside GarageBand. The sort order for the Media Browser is stored in the GarageBand Preferences file. Quitting GarageBand, logging off and on again, and then deleting the GarageBand Preferences from the User Library resets the Media Browser to the custom sort of the playlist - but that method is rather drastic and cannot really be recommended
To delete the preferences remove
from your user library.
As you probably know, the User Library is hidden since Mac OS X Lion.
You can reveal it from the Finder's "Go" menu in the main menu bar:
Finder > Go
Then hold down to options-key ⌥, until Library appears in the drop-down menu. Select it and open the Library.