Been using my iPhone 4 since late last year and upgraded to the 4s three weeks ago. I'm now having the same audio issues with most of the posters in this thread and have done tons of reading online the past few days trying to figure out the best way to move forward.
I would normally take it to my local Apple Store for at least one exchange, but since I'm working internationally, there isn't one accessible to me anytime soon. In addtion, I use my phone extensively for work; therefore, I would have to re-install various mobile security platform apps required by my company. The process of a restore or swapping phones would take a good half day, not to mention spending another hour or two reconfiguring the apps.
I am extremely disappointed with Apple's lack of action in responding to this issue. Now before you jump all over me, I do want to make it clear that from a customer services perspective, I think it's absolutely wrong. However, from a business perspective, can you blame them? They are in a heavy competitive situation with the Android platform based phones and since we represent a small percentage of total number of users who either don't have this problem or don't realize it. The truth is that it would do more damage to Apple if they came out publically and acknowledged this major issue. Think of the impact it would potentially have on their holiday sales push. This is not a small glitch, but potentially life threatening if someone can't even use their phone to call 911.
I understand some posters have suggested that we all take our phones back in exchange for a new one as to make a statement to Apple and I would agree if the issues have been confirmed to be hardware related. With the risk of getting another bad unit with the same issues, or worse, additional problems like fast battery drain or yellow screen. Not to mention spending the effort going to the Apple Store or shipping the unit back to Apple; to me, it's worth the wait until further confirmation of what the true issue is. Also, who's to say that the phone you are receiving is not just another defective phone someone else's returned with the same issue that Apple simply retested but finding no problem (because this is indeed a software issue).
Personally, I believe this is a software issue which Apple will eventually correct with software update patches. I still remember when I first purchased the iPhone 4, there were all sorts of issues with the Bluetooth performance. I looked in this forum and many others were having the same issues. Apple took the same business stance and did not acknowledge the problem. I even exchanged for two iPhones at the Apple Store and both times it fixed the issue initially, but somehow the problems came back a few days/weeks later. I finally gave up and went back to the old wired headset. It took a while, but through various software updates, I found all my Bluetooth headsets started to work fine.
One final comment, Apple really needs to get these issues fixed ASAP; otherwise, people will try other options. I have both the iPhone 4s and the Samsung Galaxy SII and I have to admit, the Android phone is growing on me. The apps are getting better and the phones are slimmer with the flexibility to interchange spare batteries. Since I need two phones for work, I originally had planned to give the Samsung to my wife once I purchased the iPhone 4s, but the Android phones have come a long ways this year from an user experience perspective. At the end, I wasn't willing to give up my Samsung, so the wife now has the iPhone 4. I hope this sends a serious message to Apple because I'm certain I'm not the only one who's beginning to prefer the Android.