Hi,
good to hear I am not alone with this problem…
Yes for the first unit I also thought maybe it was to much heat and / or some paste got into the mic which destroyed them.
For the next unit I was much more careful, reduced the heat and made sure the paste is only on the solder points.
It was working for some days I guess, but it also stops.
And the last one got broken after I’ve got asked (Respeaker lite on raspberry using I2S - #6 by SoulAuctioneer) about the wiring, where I took the next brand new unit (which I bought couple of weeks ago) and soldered the GPIO header.
It was working good and I could play a lot with it but after I decided to clean up the rest of the paste, it stopped working too, what a bad decision…
So right now, I only got one unit running for 2-3 months now and I’m really afraid of touching it 
I also think I just have to reset it someway but I have absolutely no clue how to do it… How do you reset them? I guess you are using an ESP to do it?
Here are 2 of my respeakers, where I soldered the included GPIO header on it.
PS. Today I decided to give it a last try and ordered the next unit, should arrive tomorrow…