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.