I've noticed it's harder to understand people recently, where it seemed this wasn't an issue a while ago. If I turn up the volume too much, the sound gets "blurry" and harder to understand. I've tried turning on the equalizer and using bass boost, but honestly, I don't think that affects the phone sound quality.
I've changed cases (the hard plastic cases make the sound worse) and that's helped somewhat. TPU cases or hard plastic with a soft lining really help. But, mainly, I think sometimes it's a function of the other person's cell phone that makes them hard to understand.
A person I know has a Fascinate and I consistently can't make out everything that is being said. However, that same person was surprised to learn I had them on speaker - it sounded to them as though I was holding the cell to my ear and talking directly to the mic. Another person I know has an iPhone and those calls are very clear.