Scout is relating a reminiscence or flashback. The organ scene is not happening right then and there, but rather, it's something she remembers or recalls.
"I was reminded of the ancient little organ in the chapel at Finch's Landing," she begins. From there, she explains how she would work the bellows while Atticus would play a simple song with a single finger.
Anytime an author reverts back in time in the midst of a story's plot, you can safely bet that the device being used is a flashback. While there are other names for such literary devices, in the end, "flashback" is a more universal term, used both in the literary realm as well as in modern entertainment: television series, movies, etc.
I believe the literary device being used is analogy.
Analogy is defined in Enotes' Guide to Literary Terms as "the relationship of similarity between two or more entities or a partial similarity on which a comparison is based...an example is the classic analogy between the heart and a pump". In Chapter 24 of "To Kill a Mockingbird", Scout is witnessing Mrs. Merriweather expound mindlessly about "the cooks and field hands", and when the pompous lady finally stopped to catch her breath, Scout is
"reminded of the ancient little organ in the chapel at Finch's Landing. When (she) was very small...Atticus would let (her) pump its bellows while he picked out a tune with one finger. The last note would linger as long as there was air to sustain it".
When Mrs. Merriweather ceases her prattling for a moment, Scout "judge(s)" that she, like the organ, momentarily "(has) run out of air...and (is) replenishing her supply", her unfinished thought lingering in the air until, filled by the bellows once again, the windy old lady could resume her commentary.