Can a speaker be used as a microphone?

One simple experiment you can run is to plug your headphones into a microphone input and speak into them.

Fundamentally, both emitting sound and capturing sound is about dealing with vibrations in the air. The processes are opposite (one takes electrical signals as input, and outputs air vibrations, one takes vibrations in the air and transforms them into electrical impulses).

However, consider the following:

• Both mics and speakers (or headphones) have a diaphragm, and as mentioned in the Wikipedia article:

  Microphones can be thought of as speakers in reverse

• Both incorporate circuitry able to translate vibrations into electricity (or vice versa). In other words, both have an electrical signal on one side, and vibrations on the other. The difference is what is labelled as input.

• If you think about the original phonographs, the same component was used to record and listen to sound.

So yes, a speaker can pick up vibrations (albeit sub-optimally), and a microphone can emit vibrations (albeit sub-optimally).

Many TV speakers are actually speaker arrays, too, so that you (sort of) have an array of tuned inputs (important because it might allow using post-processing to enhance the wanted audio).

This doesn't address the software question, but mechanically, a speaker can function as a microphone.

There is also "proof" that the functionality can be abused: badBIOS used the built-in PC speakers as a form of high-frequency modem—this despite the fact that one would expect PC speakers to be wired only for output. So yes, your comment was correct—there is a link to information security, and the functionality can be abused, notably to bridge air-gaps.

So this seems possible—how practical and how effective is another question, but there is precedence that it is usable for certain applications.

Edit: Thanks to @nulldev, "how practical" seems to have a fairly conclusive answer: per Arxiv, it is practical, and software solutions exist.

Speakers can't be used as a mic, but that's because they use amplifiers, which are one way only (someone please correct me if that's the case), but headphones will work as long as they use the headphone jack and aren't amplified. Though, you will need a +30dB boost for decent volume. I can confirm it by personal experience. I hooked up my 10 euro havit headphones in the mic jack and it worked, despite not having a mic wired for PCs.