When something is described as 'inaudible', it is:

The term 'inaudible' specifically refers to sounds that are so quiet that they cannot be heard by the human ear. This can happen due to a variety of reasons, such as distance from the source of sound or low volume levels. In contrast, the other choices describe sounds that can be heard, whether they are too loud, clearly audible, or moderately loud. Thus, the choice indicating that something is 'too quiet to be heard' aligns perfectly with the definition of 'inaudible', making it the correct answer.