My front microphone had completely stopped working exactly a week out of warranty. I replaced the front assembly with a new one from Fixit and afterwards it still did not work. Then I realized the microphone did pick up very soft sound if I shouted directly into it. I eventually repeated the entire disassembly and found a very thin yellow film glued to the microphone grill which had been muffling 97% of the sound. After peeling this off my microphone is working perfectly! I did not notice this on the microphone the first time and assume it was the backing for the self adhesive. Make sure you remove this - it is not obvious to spot!