Why does my media volume turns itself up and down on its own?

[AC Question]

I have the galaxy s 6 edge and it's driving me up the wall. I'm training for a marathon and use my phone for the nike running app as well as music. At first, about an hour into my runs, my phone will turn my media volume to 0 and won't turn back up. Sometimes it will turn itself back up full blast. At times I can hold the phone in my hand and watch the media volume go up and down on its own. I've tried volume lockers, but that doesn't make a difference. I turn off Bluetooth and close all other apps. Anybody have this problem and a solution?