I have now the same issue. First the people I was calling complained about loud noises, and today my microphone stopped working completely.
I noticed the same thing as @Nicolas_Rabaud, putting my charger in the phone and using it to press the bottom module towards the screen make the microphone work. As soon as I stop pressing the mic stops working. I tested with calls and a sound recording app.
I will contact support but I am also thinking of buying another phone as I already got my motherboard replaced after months of waiting (random reboots + many other issues), the screen now displays bright spots (Bright spot on the display) and my mic stopped. I thought I bought a fixable phone, not a phone which needed constant fixing. Also the point of fairphone, i.e. not trashing electronic is just completely lost. I didn’t have to change anything in my previous phones, and they lasted more than the official 5 years of Fairphone.