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Hi @matyd, welcome to our forums!
I'm so sorry to hear about your phone. 
Have you had the opportunity to try another pair of headset/headphones in your phone? Just to make sure that your headphones aren't the cause of this. If have another pair or if you can borrow a pair from a kind friend, colleague or family member.
If you have, and you are still experiencing this behaviour, I would recommend that you start with clearing data for the Smart connect app, you can do that if you go to Settings > Apps > Smart Connect > Clear data. When you are done with clearing the data, reboot your phone and see if you are noticing any difference.
If there is no change, I would recommend rebooting your phone in safe mode to rule out that a third party application is the cause of this, since these apps won't be loaded in safe mode.
Should there be no change while the phone is rebooted in safe mode, try performing a software repair using Xperia Companion.
Do note that this will delete all data stored on the internal storage in your phone so it's recommended to perform a backup using Xperia Companion prior to starting with the repair.
The last step is to contact your Local support team so they can assist you with having the phone sent in to an authorized service centre.
I understand that it's hard to be without a phone these days and I can only answer for the Nordic region where I'm located, and if you buy a phone in a store here and you need to have it sent to service they can in most cases help you with a loan phone during the period that your phone is at service. This may be different from where you are located though.
Let me know how it goes!
I see, sorry to hear that. 
I work globally, but I'm located in the Nordics so I'm afraid I won't be able to provide you with information on where the service centres are located in your market, I would recommend getting in contact with your Local support team.
They are experts on your market and will be able to assist you with any questions that you may have regading service routines, warranty, consumer laws and so forth.
Let us know how it goes and have a great weekend! 
This is definitely a Hardware issue, I have the same problem with mine for multiple plug in headphones. When you turn the 3.5mm in the port sometimes the sound will cut out, I have had my z3 for 2 years now and it has done this since back when I purchased it.
Hi @DemJ, sorry to hear about this. 
That sounds like it's related to hardware and not software. Have you made sure that there is no dust or moisture in the audio jack on your phone?
If there isn't, your device will need to be sent to an authorized service centre so it can be examined by a technician!
Your Local support team will assist you further with how to send it to an authorized service centre and answer any questions that you may have!
When I attach the headset to the device. There is no response if the headset is connected to the device. Please help me fix the device
Hi @RizkyVanDeBrown, sorry to hear about this. 
I'd recommend that you start the FM radio application in your phone and then plug in your headset again in your phone, does the message "Please insert the headset. The cord works as the radio antenna" appear when the headset is connected?
If it doesn't appear and the headset/FM radio works just fine, try clearing data for the Smart Connect application in your phone, you can do so if you go to Settings > Apps > Smart Connect > Storage > Clear data > restart your phone. This is the application that gives you the notification for when a headset is plugged in.
You can also try another headset in your phone if you have one available.
Let me know how it went!
I'm having the same issue as @RizkyVanDeBrown
FM radio does not recognise the headphones. nor in music.
Cleared data in smartconnect.
Tried a different set of headphones.
still nothing.
only started occuring after the lattest software update. everything was working perfectly til then.