Croaking sounds emit only frog males. Croaking is the way to attract females during the mating period.
In order to produce croaking sound frog takes a deep breath, closes the nostrils, mouth and push passes air from the lungs to the mouth and back. Passing through the vocal cords, the air flow inside them, they begin to vibrate, and hear the croaking.
Many frogs have a special voice pouches directly related to the mouth. When frogs sing, the bags are filled with air and inflated. As such, they serve as resonators and give a frog's croaking singing sound.