Earpiece not working - not even after soldering it into place

You see the thing is, both front facing camera and proximity works fine. But I did what you suggested and bought a new original proximity flex. Hope the phone will finally work after that. In case it is not the flex, could there be something wrong with the motherboard? I have also read about people having problems with low volume from earpiece after upgrading to iOS 8.4. And even the finger print reader seems to fail after that update.

Yes, you could have logic board damage. We don't know the history of your phone. Assuming it was not subjected to a hard drop or liquid damage, then pry damage is a big possibility. Which is why I suggested examining the connector on the board and the area around it to look for anything broken, damaged/part-removed, or completely missing. You'll need some magnification. If nothing seems wrong, then you should rule out a defective replacement part before concluding that it's the logic board which is at fault. As an example, past Thursday I received an iPhone the owner of which could not be heard during calls. I tried a new charging dock (the mic is built-in) but that didn't fix it. I examined it, nothing seemed wrong. So I tried a 2nd charging dock, still no fix. I tried a 3rd one, original/used this time, and it worked. The whole bunch I had turned out to have a defective mic. Everything else such as charging worked well..