Falling slowly, --my take on the reason is that the use of the singular (this & question) may only deal with the subject matter "sound wave" as a question. whereas the plural (these & questions) may deal with the way the question is worded-i.e. a declarative statement used as a question.
I don't know if this answers your inquiry (or if I explained it to your satisfaction) but it is the only reason I can think of.
Sorry if I added to your confusion,
Student or Learner