There are two separate volumes on your phone. One is for your ringer and the other is for media. Is your mute switch on the side of your phone turned off? Try going into settings>sounds and see if the slider for "Ringer and Alerts" is all the way up. If you turn on the switch for "Change with Buttons" You should be able to adjust your Ringer and Alerts volume with your volume buttons on the left side of your phone as long as music is not playing.
This may be an un-reparable situation but make sure the slimy black cable going from the bottom dock to the right of the spot where the battery connects. You have to make sure this clicks in perfectly. If you think this may have been damaged then I suggest buying a new dock (Follow the cable to the new part that you need. I hope this helps.
Looks like you will need a new proximity sensor. They come in a flex cable of other parts attached like the earpiece speaker, lock button and ambient light sensor. This repair may be hard. I've done two myself.