Try your headphones on a mono-source so that both sides should have the same volume. Also try to wear the headphones reversed to check if it's not your ear which is the problem.If the difference is still there, it's a problem of the headphones.
I'm an audio newbie--not sure how to get a mono-source.
Have they always been like this, or is it recent?
use the sharp part of a safety pin to scratch it all off of the surface