Water Damage Repair?

@mattbirdo The first thing to do would be to desolder the battery holder and then replace the microphone using a hot air gun. The microphone is a Knowles SPU0410LR5H-QB - https://www.digikey.co.uk/en/products/detail/knowles/SPU0410LR5H-QB/2420974. That may well fix it, but there is quite a lot of corrosion so there may be other PCB tracks that are making poor connections now.

@Alex Rogers thanks again for all the tips. I've bought a new one but occasionally get motivated to fix the old one. So, I was curious, any tips or links on how to safely remove the green cover so as to replace the microphone? Also, at the link you provide I see options for cut tape, tape and reel, and digi reel: do you know which one I would want? Thanks again for all your help

If you are hand assembling the choice of cut tape, tape and reel, and digi reel doesn't matter - this is really just an option when ordering many components as it describes the way that they are fed into the automatic pick and place machine by the PCB assembler. If you have a hot air gun you should be able to blow a bit of hot air underneath the PCB on the side opposite the two battery connectors and that will soften the glue there enough for you to lift the PCB.

Then carefully de-solder the two large battery connectors. It's best to use a solder sucker or solder braid first to remove most of the solder, and then lift the PCB slightly whilst melting the solder to remove the board.