The “who” question can be approached, investigated, and contemplated in many ways and from various angles. Some involve reasoning, others direct experience. We’re letting our experience inform our reasoning.
Before the sound of a bird or neighbor in the adjacent apartment arises in one’s mind, it had to be sounded by the bird or neighbor. Prior to that creation of the sound, the sound did not exist and, therefore, could not arise in one’s mind. The bird or neighbor “sounds”registered by one’s ear drums are by-products of various causes and conditions including bird or neighbor, how space and wind currents carry sound, the thickness of walls or glass or both between one’s self and the sound source, one not being deaf or have one’s ears muffled in some way, and so forth. Yet, these causes and conditions support the arising of a “sound” that vibrates across and against one’s ear drum or drums (both ears). None of these factors produces the designation of bird or neighbor in one’s mind, nor any preference accordingly. Name, label, preference, pleasant/unpleasant, and so forth are by-products of further (and myriad) causes and conditions including memories, imputation, perception, projection, and habits of consciousness to highlight a few.
The sensations that are within the experience of sitting on a chair are similarly processed internally. All sensations are, including refined or inner meditative one’s.
“Who” is experiencing is also a by-product, changing, arising, dissolving just as frequently as the response to a bird song and in basically the same manner. We’re being with our self and our experiences for the discoveries they impart.