I have the same problem even after replacing the speaker and the camera/proximity sensor cable. STILL doesn't work. Volume from the lower speaker still works fine, but the sound from the earpiece speaker is so low it can barely be heard with volume up, and if you turn the volume down, it can't be heard at all. I checked for missing resistors on the board and didn't see any. Any ideas from the tech world? I have done a factory reset on the phone, update to 9.0.2 on the phone, replaced the speaker, replaced the flex cable, taken the screen off a working 5C and put it on, all with identical results.
Bill