Interesting, where did you hear that?
The reality is that chameleons do not have external ear openings, a typical round window or a standard tympanic membrane, however they do have a functional inner ear, a functional equivalent to the round window and some species have modifications to the pterygoid plate that functions as a tympanic membrane. As a result, their hearing is limited, however they are able to hear both aerial sounds and mechanical vibrations. Species that lack the modified pterygoid plate that functions as a tympanic membrane have poorer sensitivity to aerial sound, however.
Chris