The phone could not hear me through the microphone, and the front-facing camera was broken. I took it to be repaired at Android phone repair shops, and lost $30 with no clear diagnosis except a vague indication that it could be the motherboard. After checking iFixit, I gathered the courage to pry around a bit myself and discovered that the socket for the front camera on the motherboard had been damaged. Under a microscope, I saw that there was some plastic thread obstructing a pin in the daughterboard ribbon cable socket on the motherboard.
Bought a replacement motherboard from iFixit, and a spudger. Kyle Libby's instructions were spot on. My phone can hear me now and the front camera works again!
Back up your phone's contents to cloud accounts, like Google. After this procedure, your phone will be back to factory settings.