The term 'Bullfrog Sound' is a reference to the sound made by a bullfrog, which is 'CROAK' (5 letters). Bullfrogs are amphibians that are found in many parts of the world, including North America, Europe, and Asia. They are known for their loud, deep croaking sound, which is used to attract mates and ward off predators. The sound is made by the bullfrog's vocal sacs, which vibrate when air is forced through them.