Same problem. For me at first the volume was fine (maybe a little bit lower than with my iphone 5, but not low enough to be bothersome). However, a few days ago I got a phone call and could barely hear the other person. I thought it was their issue but then it happened with every caller after that, the volume was so low I could barely hear them and I even had to appologize and hang up on a few callers. I also tried listening to old voicemail messages and those were barely audible as well. I tried everything mentioned in this forum and nothing worked. Resetting, restoring to default settings, deleting the phone and starting over, turning off noise cancellation, switching to mono...nothing. Then I read kali58's suggestion and thought I would just try, not really expecting it to work.
But it did work!
I dipped a Q-Tip in alcohol and rubbed it on the earpiece, then dried it with the dry end of the Q-Tip. I did it 4 or five times but it wasn't even coming out visibly dirty. Then I tried listening to a voice mail message and it did seem to sound louder, I did the Q-Tip thing a couple more times and then made a phone call and it DEFINITELY sounds normal now. Thank you kali58!