No audio after upgrade

I had a problem with an iPhone 4S six months ago that all of a sudden all of its audio functions stopped working. No speaker for anything (calls, sounds, music, Youtube, videos...), no mic for anything (sound recorder, Skype, calls, camera...), no sound on the earphone too. No sound at all. I left it away and 6 months later I turned it on and its audio was back working perfectly. So I set it up for my son and it was working fine. Until the moment I decided to update it to iOS 8.3 from 8.2. %#*@, no audio again! It worked fine for one entire month, so I don't think it's a hardware failure. It happened again only after the iOS upgrade. I have already tried to restore it in normal recovery mode and in DFU mode for nothing less than 10 times! No success... :-( The iOS downgrade from 8.3 is not available anymore. :-((

Any hints? I would appreciate any help.