Check the above tip first. Also, have you used a screen protector? i remember when I misplaced mine slightly. It really affected the volume just by having a bit of the protector covering the hole.
Another thing to check and this is a bit yuk....is there any crap stuck to the meshy bit of the speaker? I had some what I can only assume to be earwax (oops) stuck to mine which again affected the volume. I just just used a small piece of plastic to scrape everything off and it was ok after that.