The ear speaker module is most likely malfunctioning.

It is most likely a loud speaker module malfunction.

Try doing a power cycle. To do this hold the Volume Down button and then the Power key simultaneously for about 15-20 seconds. If this does not work, try holding the Volume Up button instead of the Volume Down button. Make sure to do this while the phone is not plugged into a power source. It also might take a few tries. If this continuously fails, the battery should be replaced. There is a guide on this page for that.

The loud speaker module has been damaged either by a force or water and needs to be replaced in order to hear through speaker phone, a ring tone, or music.

If you cannot hear the person on the other side of the phone talking, but you can hear them it is a good chance that the ear speaker module at the top of the phone is malfunctioning. It is recommended that you replace this piece.