I'm in the "other" category. I have very muffled audio but it's not constant; it comes and goes. I troubleshot this issue for about a week. At first I thought it was my earpiece b/c I had just bought it, so I took it back. Same issue. Then I tried an old bluetooth that I had that I knew was perfectly clear on all my Android phones. Same issue on the iPhone 4S. Tried a few other earpieces. No luck
Then I finally went to an Apple store and tried some of the phones there. Same exact issue although not as severe (even using an earpiece they had). Still had it replaced and still have the same issue. Called Apple support a few times but they just wanted to blame the earpieces. Finally got to a Senior Advisor and he basically said that I could submit a complaint and that there was nothing that he could really do on his end.
I did submit a complaint (I forget where; it was somewhere on the Apple site, probably support section or feedback). But I figure if enough people give negative feedback on this issue, Apple will have to address it. It's absolutely unacceptable that this issue has lasted for more than a year from the 4 to the 4S. Apple products are supposed to "just work". It just works alright, just not consistently and not up to the high standards that I thought Apple held themselves to and prided themselves on.