iReal Pro version 8.3.1 or higher:
In iOS 11.4, Apple made several changes to the audio settings.
In order to fix the AirPlay compatibility issues with iReal Pro, we had to change it on our end as well so currently it doesn't work with Audiobus 3 and AUM.
You should still be able to use your instruments and effects while iReal Pro plays in the background or in split screen.

If you record with GarageBand or similar apps, export your iReal Pro backing tracks in WAV format which is now a very fast process.
Then select 'Audio Copy' from the share sheet options and paste it into GarageBand.

iReal Pro version 8.2.4 and lower:
There are 2 solutions:
1. Open Audiobus/AUM, select iReal Pro as input. Select a song in iReal Pro and start playback. After that it should show up correctly in Audiobus/AUM.
2. As Audiobus pop up message recommends it, restart your device. After that open iReal Pro, then open Audiobus.