[plutonic]

You mean something like this [0]? You could use this to trick your computer/phone into using the audio jack for its microphone, but since the mic-lock passes no usable signal, you'll be better protected. It doesn't otherwise incapacitate the internal microphone, though.

[0] https://www.amazon.com/Mic-Lock-Microphone-Blocker-Pack-Surv...

[SilasX]

It would need to block the microphone itself that picks up ambient noise -- I don't think there's anything on e.g. a MacBook Air that you could plug that into and block sound.