Sentence:  Elizabeth Bishop] being compelled by the undeniable impetus of her art to break with her usual inclination to conciliate the social audience. ... she usually limited herself to a note that would not have disturbed the discreet <WSD>undersong</WSD> of conversation between strangers breakfasting at a seaside hotel. 
Meaning of the word marked with <WSD> tags:  Subordinate and underlying idea, meaning or atmosphere; undertone.