Fully turn off the phone by holding the power button and then swipping to power off. Once the phone is off, click and hold the volume button, do not click the power button, plug the phone into the computer. It should show an apple logo and then an iTunes logo. Once you see the iTunes logo you can let go of the volume button. From there you should be able to do a restore from iTunes on the computer

As for the volume buttons. Check that the cable is plugged in and not ripped cause it runs under the battery so it is possible that it was cut. They are very fragile and thing cables.

First check that WiFi is even turning on on your device. Go to settings>wifi and then it on. If it successful turns on but doesn't show any networks then walk to the nearest router. If it appears then you might just have a bad antenna. Bluetooth and wifi are on the same antenna so test both in the same way. If wifi stays greyed out and will not turn on or if Bluetooth is constantly spinning and not turning on or if a MAC address doesn't show in your settings then you might have an issue related to your WiFi Module.

Inspect the connector on the phone and on the screens. If the old one snaps in fine then it's possible that the connector on the screen is damaged.

How does it sound distorted? Like muffled or more of a popping noise? If it's muffled/quiet then it could be the earmesh. Also check if there are any rips on the front flex cable

I'd probably say that it's a bad connector. Plug everything in, let it turn on and then tap it against a surface in a couple different angles. If it turns off when you hit the phone against a surface then it's just a bad connection somewhere on the battery or phone

El error 4013 puede ser por muchas razones. Puede ser por un capacitor malo cerca de el NAND IC o puede ser que tú NAND IC tiene que ser quitado de la tarjeta madre, borrado y después reinstalado. También puede ser un cable, un charge port, el tristar. Yo primero probaría un cable nuevo, después un charge port nuevo, después probar los capacitadores cerca de el NAND por si están cortos. Si pruebas todo eso y todavía sigue, pues probablemente vas a tener que arreglar el NAND IC

I would suggest cleaning out the earmesh. That is usually the problem due to dirt or dust in the earmesh. You're going to have to remove the screen, front camera flex and then pop out the ear mesh from the front of the screen to clean it off. New screens also come with new earmeshes if you just want to replace the screen all together if your screen is cracked

First make sure that the problem is that capacitor by doing the same tests that Louis does in his videos. Check if there is continuity between both sides of the capacitor and ground. If so then there is your issue. If not then you've got another problem. First find the problem with a pretty cheap multimeter before buying all of that equipment. Cap replacements (or just a removal) is some what simple. Just watch out for plastic connectors that might be close.

Try applying some pressure down on the connector while the phone turns on. If it works fine then you just need the metal bracket. If not then you might be missing some components near the battery connector which are commonly knocked off when preforming a battery replacement.