Pair the speaker or headphones in your device's own settings. There is nothing to switch on inside iReal Pro. Bluetooth costs battery, so turn it off again after practice.

 

If the first beat of the count-in is missing, the speaker was asleep. iReal Pro plays a brief, almost inaudible sound before each song to wake it, and you can set how long that lasts, from 0 to 1500 ms. AirPods are skipped, since they do not have the problem.

 

On iPhone and iPad the setting is Settings > Wireless Wake Delay, and it starts at 600 ms. Raise it if the first beats are still clipped, or lower it to 0 on a speaker that never sleeps.

 

For the full guide, see Sound output and audio devices on the iReal Pro website.