Take note that if you are recently starting up Garageband (on Yosemite) and it prompts you to download the new sounds, make sure you update to the latest version of the Java JRE for Mac OSX. A number of people are having issues with the install process getting stuck and never completing. Apparently, this install uses the JRE and older versions of the JRE may create bugs in this part of the install.
Downloading the latest JRE before installing these sounds will help ensure it completes. (If you did get to the point where it is stuck, you should still be able to force quit the application once it does, then download and install the Latest Java JRE from Oracle, restart Garageband and redo the installation without problems)
Downloading the latest JRE before installing these sounds will help ensure it completes. (If you did get to the point where it is stuck, you should still be able to force quit the application once it does, then download and install the Latest Java JRE from Oracle, restart Garageband and redo the installation without problems)