Earpiece not working after screen replacement

I have an Iphone 4s which the earspeaker will not work after i replaced the screen. (ive replaced many many screens)

The speakerphone and headphone jack work fine.

Ive replaced the ear speaker, taken care to make sure the contacts are clean and touching, and nothing seems to work.

I have even put the motherboard into another iphone that i know to be working and i get the same result no sound from the earspeaker.....it would appear to be a hardware or software problem. Does anyone have any thoughts?