You need an AirPlay receiver on the same network as your Mac: an Apple TV, a HomePod, or any supported speaker.

 

Choose it from the volume control in the menu bar, or from iReal Pro's own Audio Output button, in the Mixer to the left of the volume slider, or under General in Preferences. With an Apple TV you can mirror the screen instead and send both picture and sound.

 

If the sound trails the position marker, you are on a first-generation receiver such as an AirPort Express. Switch on AirPlay 1 Offset in Preferences > General.

 

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