iReal Pro 2020.9 (and higher)
1 Check the volume on your computer
2 Check the volume in the Player controls panel on the right of iReal Pro (if they are hidden, View > Maximise Player Controls, or command-control-P) Increase volume: slide to the right
3 Click the Audio Output button (at the left of the Volume slider) to set your Audio routing
4 Click the Reset All Settings button in the Settings (Preferences) -> Utilities of iReal Pro
5 If you are using an external audio interface, unplug it to see if it's the cause of the problem and set the Audio Output to built-in speakers
 
iReal Pro 2020.7 and lower
1 Check the volume on your computer
2 Check the volume in the lower left corner of iReal Pro (increase volume: slide to the right)
3 Check the Audio output settings by clicking on the Audio button in the lower left, next to the Volume button
4 Click the Reset All Settings button in the Preferences -> Utilities of iReal Pro
5 If you are using an external audio interface, unplug it to see if it's the cause of the problem and make sure audio output is set to built-in speakers to check