A simple defence would be to have ultrasonic noise generators emitting enough interference to effectively jam any transmissions. It should be no more audible than the transmissions.

Of course, the average user wouldn't need or probably want this (unless they're security paranoid/enthusiasts), but it might be useful in environments where information security is essential. Maybe even 'hardened' secure devices could have built in noise generators that can't be software disabled as an extra defence feature.

It might seem simpler to just limit the frequency ranges of the built in speakers/microphones, but it doesn't eliminate the threat completely as it is still possible there could be a headset, USB sound interface or devices in the microphone and earphone jacks in use without these filters. This way, regardless of the kind of sound I/O, the surrounding area of the device is blacked out.

The whole concept of 'swearwords', IMHO is terribly outdated anyway. As someone else mentioned above, while these select words are considered taboo, their synonyms are not. Why is it okay to say 'crap' or 'poo' and not 'shit'? They mean the exact same thing. I can only imagine it was taboo to say 'shit' in public because of what it meant, but no-one seems to care about that any more. Everyone remembers it's a taboo word, but not why.