100% faulty earphones, I had exactly the same problem. After playing music for long time my left ear plug start decrease the volume of the music by it-self and the right one was always fine, to end stopping completely after 15 minutes. I was thinking is the phone or the bluetooth handset so I try it on different device and was the same. Next day it start working again like new, but again after playing music for some time it start decrease volume by it-self. It's just defective and when it get a bit hot it stopping working. I got replacement via Warranty. Do the same.
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My Evolution: Sony Xperia Z1 - UB + 4.3 Rooted + Deodexed, HTC HD2, HTC Diamond 2, HTC HD Blackstone, Nokia 7070 Prism, Qtek 9100, Nokia 3100, Motorola T720i, Nokia 3310, Sony Ericson PF 768 (Blue)
How Get Root on Sony Xperia Z1 with Locked Bootloader and the Latest Android 4.4