What specifically is the "chain" being refers to in the Christmas carol

When i was young enough to still spend a long time buttoning my shoes in the morning, i'd listen toward the hall: daddy upstairs was shaving, in the bathroom, and mother downstairs was frying the bacon. they would begin whispering back and forth to each other up and down the stairwell. my father would whistle his phrase, my mother would try to whistle, then hum hers backi drew my buttonhook in and out and listened to it -know it was "the merry widow." the difference was, their song almost floated with laughter. how different from the record, which growled from the beginning, as if the victrola were only slowly being wound up. they kept it running between them, up and down the stairs where i was now just about ready to run clattering down and show them my shoes. what is the effect of the parallelism used in the above excerpt? it establishes the rhythm of a duet to echo the song. it expresses the same ideas. it mirrors opposite ideas. it is a paradox.