A lot of times it's caused by a build up of dirt and grime in the speaker mesh. Replacing this solves the issue 9 out of 10 times.
If you can, Hold the LCD screen up to a light source and try to look through the ear speaker mesh. If you can see the light through it then its clean, but if it's blocking the light, or only allowing only a little bit of light to pass, chances are you need to replace it so that you can improve the sound quality.