Yesterday I plugged Apple earbuds into my iPhone 5 and the sound from the right speaker was very low, almost mute. Assuming something was wrong with them and having just lost the 3rd party earbuds I normally use, I bought another set of 3rd party earbuds in the evening and found them to have the same problem, but now with those the left side is almost mute and the right is fine. Of course my assumption then was my iPhone has a problem.
Now I'm at work and just tried the new 3rd party earbuds in the Mac Mini I use, and I have the same problem.
I really don't know what's going on at this point. Did the iPhone damage both sets of earbuds? Over the weekend I stayed in a house that was really cold at night, not sure if that had any effect on the phone; neither set of earbuds were in the house. I've checked all the sound settings on both devices for sound balance and such, no issues there.