Problem with ringtone volume

Hi people,

I just bought HTC ONE (M8) 6 days ago. I have big problem and HTC support dont know help me, only I must use warranty, but I cant be 30 days without phone.
My problem is that I increase volume to max with volume button and I will put phone on the table. Somebody will call me I pick up the phone and make call and after phone call my volume is down and if somebody will call me again my ringtone is down maybe on 10%. Again I increase volume up with volume button and same things happened to me... Sometimes I have ringtone to maximum 3 hours without change and sometime every 20 - 30 minutes phone will change down ringtone...

Please do you know somebody how I can fix this problem?

I tried factory settings but nothing happend...

Heeeeeelp me please

Yes I am tried turn off everything. But maybe I found what was wrong - I turn off motion gestures and all ringtones are 100% after all tests, incoming calls.

If somebody find some better solution I will very happy cause I wanna use motion gestures.

Thx