Tried to fix it by just adding a shim but it was past the point of being revived. My 12 year old son managed to well and truly kill the volume decrease function on this 3 year old 4S which I handed down to him after i upgraded to a 5S. This was replaced as part of a general overhaul (I replaced a badly scratched screen in the process as well and figured I may as well get it all done while I'm at it - the cost of parts for doing it all myself was less than taking it in.)
Off all tasks done replacing the volume strip is most likely the most complex since the ribbon needs to be glued and folded in place. It comes with adhesive strips applied. The mute button has a small screw which could easily be missed if you don't pay attention to the guide. The new strip needs to have the tips melted on the mute switch once in place. A hot soldering iron or piece of thick wire heated up will do the job.
The provided guides are good - just make sure you study all the photographs on each step before you start pulling and tugging at stuff. The microphone next to the earphone jack may tear off from the old ribbon when removing it. Pull it off with a tweezer and make sure you put the glue pad back so the new microphone will stay put.