I believe it is possible to turn a speaker into a microphone. Found a paper which claims to do just that[0]. So, there is no safety anywhere?
SPEAKE(a)R: Turn Speakers to Microphones for Fun and Profit
It is possible to manipulate the headphones (or earphones) connected to a computer, silently turning them into a pair of eavesdropping microphones - with software alone. The same is also true for some types of loudspeakers. This paper focuses on this threat in a cyber-security context. We present SPEAKE(a)R, a software that can covertly turn the headphones connected to a PC into a microphone. We present technical background and explain why most of PCs and laptops are susceptible to this type of attack. We examine an attack scenario in which malware can use a computer as an eavesdropping device, even when a microphone is not present, muted, taped, or turned off. We measure the signal quality and the effective distance, and survey the defensive countermeasures.
This only works on audio chipsets that allow pin retasking. Which is, coincidentally, all Realtek chipsets that are present in every PC...
(you also need to plug the speaker directly, mostly limiting it to headphones and laptop speakers)
Even where it works, speakers are much worse microphones that dedicated microphones, and so the amount of data that can be gathered is low. Why bother when you probably have a microphone on the same PC that can capture far more sound?
I think there was a long period where a proper PC would frequently have only the cheap stereo speakers which are small enough to far outperform raw microphone leads. But I'm not sure this works that well in >=HDMI even if some monitor speakers might otherwise be ideal.
I recall in the early or mid 2000s using some cheap earbuds plugged into the microphone port of my family computer as a pair of microphones in lieu of having a real microphone nor the money for one. Then I used Audacity to turn the terrible recording into a passable sound effect for the video games I was making.
Not knowing much about how soundcards work, I imagine it would be feasible to flash some soundcards with custom firmware to use the speaker port for input without the user knowing.
This is common at nightclubs (or was) - a DJ can use their headphones as a microphone, speaking into one channel and listening to another
Example https://m.youtube.com/watch?v=1NNP6AFkpjs
:-)
Despite this being a 2016 paper, it's worth noting that this is true in general and has been common(ish) knowledge among electrical engineers for decades. Highschoolers and undergrads in electrical engineering classes often discover this independently.
What's notable about this paper is only that they demonstrate it as a practical attack, rather than just a neat fun fact of audio engineering.
As a fun fact, an LED can also be used as a photometer. (You can verify this with just a multimeter, an LED, and a light source.) But I doubt there's any practical attack using a monitor as a photosensor.
and has been common(ish) knowledge among electrical engineers for decades.
Not only is it common knowledge it's how drive-thru kiosks work!
Source: I used to test microphone/speakers for a kiosk OEM.
Yes! LEDs as photometers is something that you don't really see around much anymore, but it is really cool. Even an LED matrix can be used as a self-illuminating proximity sensor with the right setup.