Hi @rudlberger and welcome to the forum. I note you have been reading the forum for 5min, so I hope you are aware that a) the demand is now largely on a user who manufactures new ones and b) on the option of repair.
What is wrong with the bottom module, you do not say why yopu think you need a new one? Maybe you have a #fairphoneangel near you that could help swap the module to ensure that is the problem, if it’s not obvious.
It could be the actual mic failing or the connections may be cracking up. You can remove the module and check to see if the mic is a bit wobbly.
There are some instances of repairs when it’s the poor connections etc. You may not want to try yourself but may find someone you will do a repair if that’s all it is. Replacing the mic is possible but finding a suitable one may be difficult.
The following link is to the last post in the topic, there are other posts in the topic that deal with the mic