No audio after screen replacement.

Hi there!

After replacing another screen on 4s and I have done about a dozen now, the headphone audio went bust. When making or receiving call I hear no audio from the earphone. Loudspeaker works, headphones work, light sensor works. Replaced the audio unit, cleaned the connectors, still nothing! I even put the old head piece into another iphone to check if it works and it did. So thats of the list as well. Now I'm clueless after like 6 hours of trying to fix it... :(

I would appreciate any input!

Thx,

Max