“The first time I heard a koala bellow I was genuinely amazed that an animal this small could produce such a sound,” said Benjamin Charlton, of the University of Sussex, who led the research…The pitch of a call is generally associated with a mammal’s size, because vocalisations come mainly from the larynx – an organ we sometimes refer to as our “voice box”. This organ has a valve-like opening with two lips – or folds – running across it. The vibration of these folds creates most of the sound we make when we speak.”
In case you’re curious, there is a video that shows in detail how these extra folds allow such a little animal to make such a big sound.
via BBC News