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 mas

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.