Play a sound when there are no idle entities.
If there are no idle units and the idle-unit hotkey is pressed there is now a sound played.
Beneficial when there are already units selected since selection doesn't change.
Sound courtesy of @Samualis.
With the "default settings" I could hardly hear the sound, after lowering the "music volume" and increasing the "action volume" to max the sound is better noticeable.
ffmpeg -i alarm_no_idle_unit_01.ogg -af 'volume=8' alarm_no_idle_unit_01.ogg