Volume and mute button not working after screen replacement

After replacing my screen, all is working except the volume and mute buttons. The iphone asumes that it is muted, so when receiving a call, no sound is played. The speakers and headphone are working like they should work, just that the phone is locked in mute mode. When pushing the volume button or switching the mute button, nothing happens. Does anybody have an idea what went wrong and how this could be fixed?