Hi there!
On the iPhone 5s when you replace the screen, you are transferring a few things from one screen to the other, front camera, home button, that nifty shield, the proximity sensor, and the secondary mic! it looks like a little golden box with a peg sticking out of it, it is meant to push right inside of the little rubber circle next to where you put the ear speaker. That bad boy is probably blocked by something. Try taking it back apart and examining that part of the front camera flex assembly. When you put it back together, take extra special care that the little golden peg is pushed inside of that rubber circle just right!
-Samantha Stout
Fixin' electronics since 2002