iPhone 4S microphone ain't working properly

Hallo, I have a waterdamaged iPhone and have repaired it almost... their is just one problem left with the sound from the microphone. During a call, the other sight can.t nearly hear me and I have to talk almost in the microphone before they can hear me.
I have changed the logic board with a full working 4s and still the sound from the mic is terrible. So it is clearly to me I have to deal with a hardware issue.
I hope someone has a tip or trick for me, so I can repair the 4s

Ps sorry for my bad English, I'm a Dutchman