People were complaining to me about sounding muffled/echoing. They sounded fine. I read as much as I could find about the problem. Many people will tell you to change phone settings, and that's great if you are willing to do that and it works. However, my solution has been to remember to reboot the phone at least every 3 days. Hopefully 2.1 will fix this.
If you are on a call and people say that they can't hear you, try toggling speakerphone on and off. This has always cleared up the problem for me. Time for a reboot.