Should be possible to check if its the thermal mechanism by looking at the frequency response. Should act like a low pass filter because of the mics thermal mass. If the response is flat i agree with Horrowitz that its probably leaking into chip packaging. The fact that IR light still works suggests to me that it's the mic heating up.
If you have a third party controlled voice device online in your home that controls anything other than the lights and what music to play, you're a fool who deserves what you get.
No consumer IOT device is secure enough to totally control either your safety or security, period.
...whoa there, pardner. Nobody deserves to be the victim of burglary/robbery simply for being uninformed or naïve. Relatively few people really understand the implications of this sort of technology. I don't hold that against them. I take it as a learning/teaching opportunity instead.
Besides, who the hell would've guessed you could work these things silently and from long range with lasers?