I have the same issue – what a bummer. The newest and most expensive smartphone from my carrier, yet something as elementary as quality sound quality is not something I can expect.
If it had been a problem with this particular unit – no problem – sh*t happens. But obviously it is not "just this phone". From the many hundreds of internet postings I´ve found, this has to be a hardware or software related problem, that Apple should address and fix.
I have been Apple religiuos from my first Macintosh SE, through pizzabox machines, generations of Newtons, iPods, iPads, -Books and Pros and Phones – and I am sorry to say that I think the quality is going down. Apple has never been good at "listening to their customers", but with the exception of a few hickups, I have always felt the products have lived up to high quality standards. With the iPhone 5, there has been several signs that things have changed – and not necessarily for the better.
Can I live with a smartphone that drops calls and with a sound quality that makes conversation impossible, unless I am within 100 yards of a mast? No way.
Time to take a serious look at this, Apple – and come up with a fix.