Hi @AndrewDeere, let me see if I can help out here.
Have you been able to test the functionality of the headphone jack in safe mode? Safe mode is a troubleshooting mode where all apps downloaded from Play Store is temporarily deactivated, thus you can see if this behaviour is caused by a downloaded app. For more information on safe mode and how to use it, click here.
If the behaviour persists in safe mode, I suggest that you try to repair the software on your phone using a computer and Xperia Companion. Keep in mind though that a software repair will erase all user data, so remember to do a backup of your data before the repair.
If none of these measures manages to improve the functionality of the headphone jack, I would advise that you get in contact with your local support team in order to have your phone sent to an authorized repair center. If you have any questions regarding the service such as routines, warranty and consumer laws, these kind of questions are best answered by your local support team as well.
Try these measures and let me know how it goes!
