If you can't fix it on the hardware side, there is a way you can do in the program side.
Install iFile or other compatible apps.
go to
/System/Library/LaunchDaemons
find
com.apple.iapd.plist
delete it.
There you go. you should get back you volume bar again.
Thanks to a dude from the web that found the solution.
Hope it will help you beside hard working on your iphone's hardwares without any solution.