I understand that the use of speed brakes can be unsettling, but it can be done in such a way to minimize discomfort and alarm. When ever I feel the need to use them I open them slowly so that the passengers are not alarmed by the initial buffeting they cause.
Additionally the airline I work for English is a second language and since I don't speak the language of the airline I work for making a PA announcement would require both pilots taking the time to make a PA in two different languages, again time consuming and distracting, time might be better spent listening to what ATC might want you to do.
In the future I will keep in mind your thoughts and see how I might be able to inform the passengers in a timely manner that the rumbling they might feel in the descent is entirely normal.
Cheers!
