i have a similar issue, two of my customers complained that their screens went black after i replaced their displays. the phone works perfectly, but it shows no picture. i always use best quality grade a+ displays and i have never encountered this issue before none of the phones were on ios 13 though
are you 100% sure the battery is the correct model? that might be what’s wrong. otherwise, check if any of the metal pins on the fpc connector is damaged. make sure to align the cable above the connector perfectly. it may take some time, as it can be really tricky
If you press the back of the phone against your ear, do you hear a hissing noice? similar to the one you are experiencing while recording? that might be the issue. many smartphones emit a weak hissing noice from within, but is is significant on some models. otherwise, do a test recording with a new front camera flex plugged in. many smartphone repair shops will test this for you for free.