iPhone 5S sounds muffled

Yes - I am having this issue!

I am searching the forums now to see if I should return but it sounds like that won't help.

I experience the identical issue, even after the Apple Store replaced my phone two days ago (followed by a "new phone" rather than a "restore from backup" configuration). I've tried turning off/on Phone Noise Cancelation and manually covering the video mic during a phone call without success, but switching to the right ear helps as reported by those with whom I'm speaking. I always keep the volume set to full, so maybe that's part of the problem. I'm still researching this to see if it's possibly a hardware design issue (i.e., the location of the mics/speakers) or possibly some type of software conflict. I'm not using Control Center's AirDrop, but I see that there's an AirPlay button to toggle between iPhone Speaker and iPhone, and mine is set on iPhone. Is that the correct setting so as not to conflict with phone calls? As much as I appreciate the incredible technology integrated into the iPhone, foremost it needs to be a working phone.

I'm having this exact same issue with my work iphone 5c, people tell me I sound like the adults from the old Peanuts cartoon and as soon as I switch to my right ear they say its way better and can understand me again.

Remove the placstic screen protector that comes on new iphones. Or if you are using a proective case, take that off and see if that helps. And make sure you are using a case that is made for you specific iPhone model. Only use 5c cases on a 5c. and son on.

The same problem here... no screen protector, no case... simply switching from the left side of my face to the right side improves the ability for people to hear me... Unbelievable, it has to be the way the mic is positioned or the device...

I have also switched out my phone and it was no help. I never had this problem with the Iphone4 and it is a huge problem because unless the people at the other end tell you that there is a problem you don't know. I have never seen a response from Apple acknowledging the problem including at the Apple store when they switched it out.

FWIW. I've always used my left ear when using any phone (including my iPhone 3 and 4) and have held the phone nearly perpendicular to the ground (i.e., at a 90 degree angle), and no one ever complained about echoes, muffled sound, etc. Once I realized that the 5S is longer than previous versions and that, as a result, the microphone is farther away from my face, I adjusted the way I held the phone (still using my left ear), making sure that the microphone is pointed at my mouth (almost a 45 degree angle instead of the 90 degree angle I was accustomed to using). Since I've done that, I've had no complaints from those on the other end of the conversation. It took awhile for me to remember to make the adjustment, but that seems to have addressed the issue for me, and I can still use my left ear for phone calls. I'd be curious to know if this seems to address the issue for anyone else...