A known issue with GarageBand's rendering causes NOISE tracks in exported GarageBand projects to be silent. We hope this will be resolved in a future update. 

In the meantime, to export the audio of your GarageBand projects you may record the audio output from your iOS device:

  • Connect to a MacOS device via Lightning cable. 
  • On the MacOS device, launch QuickTime Player. 
  • Select File > New Audio Recording. 
  • In the drop-down menu accessible via the arrow to the right of the record button, select the iOS device.
  • Click the record button to begin recording the audio output of your iOS device.

You may also record from your iOS device to a desktop DAW. To record into Ableton Live, for example, see this article.