Let the sensation of breathing be like a sound, simply appearing, and by its mere appearance it articulates this space of consciousness
Here, Sam Harris uses a "trick" in order to reinforce the notion that the breath is just appearing, rather than being an intended activity, a doing by a subject or Self.
The "receiving" mode of perception is easier done with the sense of hearing than with breathing. That is because sounds generally come from the outside, and thus are felt to be passively "received". There is no muscular activity involved. And, in several meditations Sam Harris points out that we cannot block or produce sound.
Also the sense of seeing is much more "contaminated" by this sense of Self, at least when we focus on something. Then, there is a strong perceptual subject-object split.
This is shown in the following diagram.
It is well known that the sense of being an isolated Self diminishes when we go into peripheral viewing mode where we perceived less conceptual.
So, Sam Harris uses the analogy from breathing to hearing to reinforce a non-dual view by reducing the sense of being a doer.