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
Did you actually hold the power and home at the same time for 10 seconds? It's not a quick thing, but needs to be held for 10 seconds (or close to it). If that still doesn't reset it, and you have replaced the LCD, then you're looking at a bad connector or bad chip on the board.
Certainly possible. Since batteries are pretty inexpensive, you can buy another to test. Not much to drain the battery on a camera like that other than normal use. If the screen is overly bright on it and you're leaving it on a lot, that would cause a drain.