So you're saying that you can only use loud speaker and not the earphone speaker at the top? The iPhone 6 has 3 different microphones, one on the bottom, one next to the camera on the screen and one next to the back camera.. I know siri uses the front mic. Try using siri and see if it understands you, also try taking a front facing video and see if you can hear anything back in the video.. If not then it sounds like whoever replaced the screen didn't install the front camera, prox sensor and mic flex cable or it may be damaged which would explain why sound recorder wouldn't work. It may also just be a software issue. Have you tried restoring it with iTunes?