A previous known issue with GarageBand's rendering caused some NOISE tracks in exported GarageBand projects to be silent. This has been resolved, so if you're experiencing this behavior, make sure that you've updated NOISE and GarageBand iOS to the latest versions.
As an alternative, 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.