Well, then it’s probably about the bottom module that includes the primary microphone (the 1 mm hole right next to the USB port). The bottom module has been the most failure-prone module in the FP2, so your microphone might not be firmly soldered inside the module anymore.
You can still unscrew the bottom module from the core module and screw it firmly back into place and see if it helps, but probably it will take a new bottom module. If there is a Fairphone Angel nearby (or a friend or someone with another FP2), you can also swap the bottom modules for testing before you decide whether you need a new bottom module.