I was tempted to go with the "some device on the path to speaker with a lifetime timer and a warning buzzer" idea but you reminded me of a job a long time ago w

I was tempted to go with the "some device on the path to speaker with a lifetime timer and a warning buzzer" idea but you reminded me of a job a long time ago where we wired pre-cat5 serial up to a Dec-10 replacing a board, and phantom ascii started to appear on the input terminals. Sometimes, RFI just makes the right shape to fool things and you have valid bitstream from .. nothing. So if there is an element on the path to the speaker which is the right tuned antenna to something either periodic on the ISS, or periodic as the ISS moves in orbit, you could get something from .. nothing.