Xperia Ultra Z faulty Sony won't repair/replace

I bought an Xperia Ultra Z four months ago.<br> The sound has always been unreliable, it cuts out several times a day and needs several reboots before the sound comes back. Also the phone will buzz uncontrollably for no reason and won't stop buzzing till I turn it off. The phone is unusable, I can't make or receive phone calls as the sound cuts out every time. Nor can I play music or watch movies or use any apps with sound. Software updates do not fix it.<br> I have sent it back for repair three times, refs:WB-SE-***6,***,***. The first two times they replaced parts and sent it back with exactly the same problems as before. The final time I have received an email saying they cannot repair under warranty and inviting me to buy another phone or pay to have the faulty phone returned. There is no explanation with the email as to why the phone cannot be repaired under warranty - maybe it was because this time I enclosed a note with the phone asking them to actually fix it this time and test before sending back or else give me a replacement. Please repair my phone or replace it for free under warranty this service is appalling and insulting.