The speakers have to be lined up with what are really pretty tiny cut outs in the front glass. There is probably some sort of foam gasket between the speaker and the glass of the screen. The gasket, if there is one, not only directs sound to the cut out, but also sometimes serves as a sort of tiny megaphone and is integral to the acoustics of the speaker.
It is easy to have a slightly misaligned gasked blocking sound, which can have a surprisingly significant effect on the sound quality and volume of a cell phone speaker. So, although I'm just guessing, I bet something like this is the problem (rather than the speaker element itself).
I've seen this sort of thing badly put together in other phones. So it's probaby a quality control issue in the assembly of the phone.