I have issues with jack too. When you pulg in headphones, it sometimes wont recognize that something was plugged in, but if you try to twist the jack in the phone, it may get working (the sound is clear, no loss). The problem is when you put the phone in a pocket and walk, the jack is moving there so it is very likely to stop working and u have to twist it again.
When I bought the phone, I had no problems with it, but after first water test (which resulted fine, there was no water inside), the problem started.
Got the phone for 2 months and the issue still persists.
My solution is plugging in, search for the right spot before it starts worrking and walk with phone in a hand, holding the cable so it wont move in the jack.