Mic not working on videos and FaceTime

Hi have got a iPhone 8 Plus, the mic works fine for phone calls, on both handset and loudspeaker, but as soon as you try making a Facebook call, video or make a audio recoding no sound gets played or picked up at all. Have tried replaceing the front camera flex as thought it was this mic but tried a couple of known working ones but no joy, have also replaced the bottom mics with the charging port with no joy