There was an article (a while ago) showing that at least one major brand of onboard sound cards has undocumented features to switch the audio-out jack port to input. Cheap headphone/earbuds function as a microphone too, so that's a thing. Only if you leave the earbuds plugged in, of course. It can't listen from an empty jack port.

I (personally) think that intelligence/privacy-wise the mic on many devices is a lot more troubling than a camera. Camera is very directional and (again, personally for me) would at most result in embarrassment? While a microphone can record all conversations in a room/appartment, transmit everything with fairly negligible bandwidth, and taping over it won't do much good. You actually need to open the device and disconnect/cut a wire.