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Hi @Flender, I'm sorry to hear about your phone and I understand this has to very frustrating. 
Does this happen only when you walk or does it happen when the phone is put on for example, a table or if you are holding it in your hand?
Could you please try and use the same headset when either holding your phone in your hand or putting it on a table and see if you experience the same behaviour?
If you are using a case try to remove it and see if there is any change when you walk with your phone.
Let me know how it went! 
I have the same issue :
When I play music with deezer App for instance (with jack or by bluetooth), the 3 or 4 first musics are played with correct volume. But After some times, the volume down automatically.
I must do pause and play to have the correct volume.
And it starts again to be down after 3 or 4 music...
(no case, and the problem occurs even when the phone is on a table)
Hi @sbouget, sorry to hear about this. 
You mention that you have to pause and play the song again to have the same volume as the first 3 or 4 songs, is the volume still at the same level as the first songs when you adjust it with the volume keys on either your headset(s) or the buttons on the side of your phone?
Have you tried and see if this occurs when using the loud speaker in your? If you haven't, I would recommend doing so.
You can also try and play a couple of videos/songs in the Youtube application using your headset (both Bluetooth and cable) and the loud speaker and see if you experience the same behaviour that the volume is turned down automatically after a couple of songs.
Let me know how it went!
HI
Please do help me i am facing the same problem with my Xperia Z5
The sounds automatically goes to none
Thanks
Hi @moaiz3, sorry to hear about this. 
Could you please get back to me with some further information? For example, if this only occurs when using a headset?
You can also try restarting your device in safe mode, so we can make sure that a downloaded application isn't the cause of this.
Let me know how it goes! 
It occurs when ever I'm playing any audio with and without Earphones either I'm listening to audio files or YouTube even in whatsapp audio
Okay, Sure will try the safe mode
Thanks
Thank you for the information and you're welcome! 
Have you had the chance to try your device in safe mode yet? If you have, was there any change?
Keep me posted!