1. Do you have Original Apple Cable ? 2. Did You try to enter into DFU Mode while connecting iP to Comp 3. Did You Tried to connect Your phone with Comp when screen is pluuged off ? 4. I can suggest You to connect it to another fully working CPU, sometimes it help 5. Was it Your first screen replacement ?
Hello. In my opinion it's not volume flex damaged. But ive got questions for You : 1) Does your phone getting lag when trying to rec a video, or trying to watch any video with sound on YT 2) Does Phone lag while some1 calling you, when you have sounds turn on , and does he works properly when you have sounds off ? When phone works properly on headphones You could have damaged sound IC, it's very expensive in repair, and very dangerous, beacause its "under" screen connections on Mainboard.
Everything is possible, but You have to project your own iOS, lightning flex, and PROBABLY new paths on maiboard :) You can do it, it only depends of that, how rich you are, and how smart people you will ask for this.
i Can't stand posts when someone asking someone to connect water damaged phone to any kind of voltage... After Water Damage, You have to turn off Your phone, do not try to charge or any factory reset. Only thing what you do is bring Your iPhone to any kind of service, which have an ultrasonic bath, and after the process of cleaning logic board with IPA, and ofcorse drying Logic Board, you can try to turn it ON iPhone IS NOT WATERPROOF in that sense what people thing, it's only commercial.
Hello. I'm glad to can help You :) Take your phone to the first service, and ask them to reconnect the screen flex cables if that will not solved your problem, then You will have to buy a new LCD. Anyway things like this IN MY OPINION cant be caused by screen protection, probably some kind of fall.
There's nothing speacial in replacement the back housing, all You need is patient, some kind of hot air, and remember. STAY ORGANISED, and do not follow everything what You see on YT or some kind of it before you watch it to the end :)
Probably microphone isn't in correct position, it have to be putted correctly in to hole, if it will be not, youre callers may have problems while calling You like their own voices in their earspeakers. There's low chance to have damaged 3 mics (everything is possible, you don't know who and how disconected those flex from phone - maybe some1 used to high temperature and melt the speaker), but You can also try use a IPA and toothbrush and gently clear the port on motherboard.
All waterproof phones with a high parameters are 3 time thicker than iphone 6s :) Btw. theres no phone what is fully waterproof, their are only a lil bit more resistant on water than for example iphone.