Final answer:
The conversation between the two ladies is about an organist's performance involving specific organ stops and technical choices while playing a pipe organ.
Step-by-step explanation:
The musical activity being talked about by the two crabby old ladies at a restaurant on a Sunday involves playing a pipe organ. The terms mentioned, such as nasard, bombarde, and mixture, are specific organ stops which are used to control the sound of a pipe organ. A prelude is a musical introduction, further indicating that the subject is organ music. The term cipher refers to a malfunction where an organ pipe continues to sound even when the keys are released. The conversation implies a certain level of dismay at the organist's choices during a performance.