More info might help. Is the 'phone connection a straight cable in or a split to left and right. Have you double checked all cable connections including any into the 'phone itself? Is the material your are listening to Stereo and not Mono? What are the 'phones plugged into? Have you double checked your work (preferably with a mulitmeter). I suspect you may have overheated the connector with the soldering iron, but unless there is some compensatory capable circuitry in the 'phone I don't see the mono to 2 channel thing happening. Are they powered 'phones (al la Noise cancelling)? Also if you are plugged into a computer have you checked the settings to ensure you have not accidentally changed something.
Mono I get, into both ears is odd and suggests a short between L and R and not to ground (or a source issue). Is the volume significantly lower or higher than you would expect?
Do you have another set of 'phones lying around? buds even, something you can use to prove out it is the 'phones.