Strange as it may sound, when people say that switch the side of your head you are holding the phone against and see if it improves... in certain instances the noise cancelling microphones don't work right and changing the orientation helps.
Also, even stranger, a co-worker with long hair had similar issues with her Rezound, but if she had her hair tied back everyone could hear her fine, if it was down and covering the phone, she sounded garbled. She got a Motorola Razr Maxx after two replacement Rezounds and had no more issues. Also probably goes back to the noise cancelling mic placement.
And simply checking that all the microphone openings are clear of debris and bits of plastic from a cover or shifted cover. There is one on the top, left hand side of the phone and one on the bottom.
Just some simple things you can start with.