Look at the hole in your screen, where there is a small piece of black mesh.
Behind that mesh there are the ear-speaker from which you listen when phone is near to your ear AND the mic from which you talk when in loudspeaker mode.
If this issue happened some time after your replaced the screen, and the sound became lower and lower with time, there is probably some dirt on the mesh. Brush it clean with a toothbrush dipped in iso propyl alcohol (90%+) or even scrap with a needle and then punch 3-4 tiny holes.
If this happened immediately after you replaced the screen and you can't hear any sound at all, then that cable is damaged; or you did not click it in place properly; or there is a small piece of film covering the hole that you failed to remove before transferring the cable.