Work down this list on Mac or Windows, and stop when the sound comes back.

 

1. Check the computer's own volume. 2. Check the volume slider in the player controls; if they are hidden, press Cmd-Ctrl-P on Mac or Ctrl-Shift-P on Windows. 3. Check the Audio Output, either in the Mixer beside the volume slider or under General in Preferences or Settings, and make sure the sound is going somewhere you can hear it. 4. Unplug any external audio interface and set the output back to the built-in speakers, since an interface that is powered off or claimed by another app is the usual culprit.

 

As a last resort, use Reset All Settings, in Preferences > General on Mac or Settings > Utilities on Windows. It resets your preferences, not your songs.

 

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