It's a bad solder joint. I know, you looked at them and they look fine. Reflow the joints. If you search the forum you will find this issue posted every single week. And the solution is the same every single week - reflow your solder joints.
Didn't fix it? That's because you missed soldering the inner terminal closest to the chassis on the headphone jack, where the black wire passes thru. Really. It happens all the time. You can verify this, just look through a few weeks worth of posts. I know it looks fine, but you need to reflow it.
