Recently my headphones are playing both left and right channels over both ear phones. I was wondering what the cause is and possible fixes. They are no longer in warranty.
 
I tried using both on board Realtek HD as well as a usb sound card (ASUS Xonar U3), and running spdif out to my blu ray player.
 
Computer Specs
CPU: Intel Core2Duo E8500 @ 3.2Ghz
Mobo: Gigabyte EP45-UD3L
GPU: Nvidia 660 Ti
Ram: 5GB
Windows 7 Pro 64bit
 
Drivers:
Realtek High Definition Audio Driver Version 6.0.1.6662 (From the gigabyte website)
Asus Xonar U3 Audio Version 7.12.8.2157 (From ASUS website)
BluRay Player Link http://www.insigniaproducts.com/products/blu-ray-dvd-players-recorders/NS-BRHTIB.html
 
 
If soldering skills are required, I do have the ability and knowledge to do it. I don't really want to send them to the repair center for NA, because they are out of warranty and I feel it would be very expensive.

Check the circuits in the left cup.  Find the connections for the wiring and make sure there are no flux bridges.  A tiny screw driver will do for scratching the flux off carefully, you won't need much force.
 
If that doesn't fix the problem, then you're going to have to repair the circuitry.  I assume that you will need the service manual in order to know what voltages you should be getting at various points in the circuit and/or what values those tiny little SMD parts are so you can check what has failed in order to replace the part(s).  In which case, I would just send it in for repair if I were you. 

I should have mentioned: follow DT770 recable/disassembly guides.

Just realized that this is the same problem I had 2 years ago. Except now it's not in warranty. Figured after I got them repaired I'd have no problems.
 
http://www.head-fi.org/t/613329/mono-sound-over-stereo-headphones#post_8624781

Circuitry eventually fails.  Circuit components don't last forever.  A regular DT770 could last you far, far longer but won't have a boom mic without you getting a separate one.