First, make sure that the volume is turned all the way up. This sounds like a no brainer, but I've found that sometimes when I'm on a call with the phone up to my ear, the volume button doesn't seem to work--I have to take the phone away from my ear so that the screen comes on, then adjust the volume. You should be able to see the volume bar when you do this. So perhaps you think you've adjusted your volume all the way up, but in fact it hasn't budged?
Second, you could try turning off Voice Privacy under Phone->Settings->Voice Privacy. After doing a little searching on the web, it seems that most authorities don't think turning it off should affect voice quality, but there are a few people on forums who have noticed some improvement (mostly with echoey sound quality). I don't pretend to understand the technology, but it sounds like the Voice Privacy option is somewhat obsolete anyway, so turning it off probably isn't a big deal (at least as a trial).