No sim most likely is the sim detect switch at the back of the sim card slot. Open and see if inserting the sim breakers the circuit. If it doesn't break the connection you can bend the pin out a Little so the back if the sim tray pushes it further. Very very carefully!!!!
Quick way of identifying problem. Open and short home button pin pads on dock. If this works then you know it's a home button flex cable or home button pin problem. If it doesn't work the it's the dock or dock connector, or the dock socket.
9/10 times this program can be fixed by replacing the dock connector. If that doesn't fix it and it's muffled check that the new rubber insert isn't clogged. A good test is using the mic without the rubber attached then with it attached. Also the mic has double sided tape to adhere it to the rubber if the old adhesive is still inside it could be blocking it. $12 from www.iphonepartsaustralia.com
You only need one screw and it's the longest one for the wifi plate. Also there is a little brass clip that contacts the motherboard. Contact www.iphonepartsaustralia.com they should be able to mail one for less than $10