Considerable amounts of money and expertise have been thrown at sound analysis, and it's used in a range of situations from telling what submarine is in your waters to knowing what a typist is typing.
And eventually people put parallel ports on typewriters because typists are more expensive than a parallel port, at which point you've got a cumbersome but very hackable printer.
Researchers recover typed text from audio recording: https://news.ycombinator.com/item?id=7448976
Acoustic snooping: https://freedom-to-tinker.com/2005/09/09/acoustic-snooping-t...
And Germany was thinking of trying exactly this: http://www.telegraph.co.uk/news/worldnews/europe/germany/109...
Algorithm recovers speech from the vibrations of a potato-chip bag filmed through soundproof glass - http://news.mit.edu/2014/algorithm-recovers-speech-from-vibr...
The gist is that typewriters aren't really that much more safe. You still have a weak human element, and it's also easy to bug a typewriting machine.
This story just gets more interesting all the time.
"... in 1985 Soviet spies installed secret "keystroke loggers" and antennas in at least 13 typewriters in the US Embassy in Moscow to detect and transmit the typing patterns of embassy secretaries. "
You know, classic, tangible, "built to last" stuff.
It would be amazing if they went back to typewriters as being 'unhackable' devices.