The croaking that we hear is done by male frogs. Males call mainly for 2 reasons: either to attract females that are ready to mate or to repel other males from a territory.

When calling, a frog forces air from its lungs through the larynx, causing the vocal cords to vibrate and produce the sound. The sound is then amplified by the vocal sacs.

Vocal sacs are pouches of skin beneath the floor of the mouth When calling, a frog keeps nostrils and mouth closed and uses muscles of the body wall and throat to shunt air back and forth between the mouth-sac cavity and the lungs.

Nikon D200 with Sigma 70-300mm APO DG Macro 1/125sec at f10 ISO 320, Flash fired.

References: Enchyclopedia of Reptiles & Amphibians - Dr. Harold G. Cogger & Dr Richard G. Zweifel