GarageBand for iOS: No audio when monitoring external device
After enabling monitoring for an external device in the Guitar Amp, audio output from GarageBand may stop. This can happen when the iPad's audio output is set to Mono Audio in the Accessibility settings.
To resolve the issue, tap Settings > General > Accessibility, and then turn Mono Audio off.
Alternatively, in Settings tap GarageBand and turn Guitar Amp Crosstalk Protection off. However, you may experience unwanted feedback with Crosstalk Protection off.