It seems like something that would work for built-in speakers too. They're driven by the same audio codec chips (eg realtek), but wired directly instead of through an external socket.
I think the opposite. Speakers are driven by an amplifier that will not let the audio in reverse, even if the soundcard output was switched to act as an input.
But might there be sufficient coupling between the amplifier's output and its input, such that driving the output pins via speaking into the speaker can produce a voltage on what is usually the input line to the amplifier?
I suspect it may be possible, depending on what the firmware allows - amplifier circuitry often utilizes feedback in some form or another, which is essentially coupling the output to the input. And even if the amplifier is powered off, there's bound to be capacitive coupling between the input/output lines, but that's probably too small to be useful.
While I have seen a soundcard using two such inverting stages in series for headphone output, in laptops dedicated chips are more common which probably don't have such strong coupling. But better safe than sorry if the NSA is after you :)
