I've just fixed the same issue and want to share my experience. In my case the problem was in the flex cable. Apparently, I damaged it while changing a digitizer. I assume that I could have bought the cable and replaced it, but since I was not sure in the source of the problem I simply cut the piece with the volume up button out of the flex cable. Now everything works! Volume can be controlled with an app. I'm fine with a little bit crippled phone.