I had the same problem on RomAur 6.1 stock KK. To fix it I just reverted back to CM11 and jack worked perfectly and then I went back to RomAur and everything was fine.
It could be a software issue I recommend reflashing the ROM if problem still persists flash another ROM.
If you're still facing the same problem then go full stock with locked boot loader and send to Sony.
Also did you apply any volume mods cause I'm trying to figure out why it did that?
Sent from my C6903 using XDA Premium 4 mobile app