This may not prove anything with my sample set being small but 3/3 AT&T demo units I tried all sound good in store (i.e. 3 different stores). 2/2 verizon demo units in store sounded bad (or marginal, certainly not as good as the AT&T ones). Geographic location has no apparent influence (I tested locally, and I went to a mall about 15 miles away and tested verizon and at&t in same mall). Plus, AT&T phones all had more comfortable vibrate (smoother, less intense haptic feedback on stock keyboard).
It could be AT&T actually tests their demo phones first before putting them on the floor. So again this may not mean anything. I know people also have problems on AT&T but I find this interesting anyway. I do wonder if verizon has a worse or more frequent of a problem or not. Too hard to tell from forum posts. Again to be clear, I call my voicemail, record message, and play it back on a landline and compare quality and loudness. Verizon are all quieter and harder to understand. If I was able to buy the working AT&T phone and put on verizon I would be happy.