My wife's new Iphone 5c is doing the same exact thing and just can't figure it out. Took it back to the Verizon store and they thought it was the port had fuzz in it. They were wrong, its still doing it. I don't know if its a noise canceling thing or if its a microphone placement issue with how she talks on it. It gets frustrating to keep asking her to repeat things when I can't understand her at times.
Hopefully someone can give us some help with this issue.