There's at least a pre amp and a d/a converter that the mic needs to have powered up and running before any of it's signal can be used by the rest of the computer.
Which will all be inside an IC, away from the microphone.
So what, does the light turn on whenever the entire IC sees power? Because if it relies on a signal from within the software-controlled IC, that's not exactly trivial to audit.
Not the last time I went down this road hardening a Thinkpad, see Step 6 at [1] (not my site). I haven't bothered with any smartphones yet, so I can't speak to what they tend to use for a mic. But if I carried one I'd definitely rip out all the voice coils and rely on a pair of headphones w/integrated mic for voice comms to mitigate potential for eavesdropping.
When replacing the lightning assembly on my iPhone 6, I noticed that the microphone looks passive. Here’s a photo[0]; It’s the little bronze(?) square next to the headphone jack.
So what, does the light turn on whenever the entire IC sees power? Because if it relies on a signal from within the software-controlled IC, that's not exactly trivial to audit.