Echo after screen repair

It depends on the ROM. Most newer 4.2 ROMs have already fixed the issue but try turning on Noise Suppression within the dialer's settings. Go to phone, click the menu button, then navigate in settings and scroll down until 'Noise Suppression' shows up. Tick the box to check mark it and the echo should be gone. Noise suppression also helps with complaints about muffled sound.

In my case in LG G2 mini, there was the same problem with echo. Thought it was damaged speaker's seal, but it occurred that I pushed motherboard too hard between jack and speaker contacts. In opposite case there is a small rubber seal that touches microphone on motherboard and if board is placed without pushing, that plastic case with antennas isolate microphone with that rubber seal and echo is gone. Maybe in this case it would be applicable.

There is a tinny rubber baffle that isolates the sound from the pin hole to the mic that has to be moved over when the screen repair is done. Not doing so will cause sound to bounce around the housing of the phone before the sound gets to mic. This also applies to the noise cancellation/ambiance mic. I come across lots of phones that have this issue. Particularly LG G3 that just had screen repairs.