After performing the iphone 6s screen replacement, it is very likely to mess up the camera because of the new screen you put on. Try changing different screen and the camera itself which should fix the issue. Still not working then try resetting the phone or just try updating the ios on your iphone

this is definitely the issue with the audio ic. replacing the audio ic should fix it. most of the iphone repair store don't offer this service since it's not an easy job to do.