When I spoke on my iPhone 4, people on the other end of the line could barely hear me. I thought the main microphone could be defective.
This was the second time I took this phone apart. Previously, I replaced the battery and the home button. Unfortunately, during this repair I broke off two of the four EMI fingers on the speaker assembly. Perhaps they were already fatigued from my previous repair. Fortunately, everything else went together OK. The phone works and the microphone problem is fixed. I'm not sure if the loss of the two EMI fingers has degraded the phone's performance, but it seems to work as well as ever. The repair took me about 2.5 hours.
Be careful when reassembling the speaker assembly. The EMI fingers are remarkably brittle and delicate. Also, use a muffin tin or a tray with multiple compartments to hold all of the tiny components in the order they are disassembled. This makes reassembly much easier.