It could be a notification with no sound assigned (or sound disabled) to it that's going off in the background. I experienced a similar problem where music would play at a lower volume and stay there. So I disabled all notifications except for a few I really need, and the problem went away.Whenever I start a video on my 6, the volume will decrease after a few seconds and then go back up.
My old iphone 5 did this all the time and I thought it was just an issue with the phone.
Anyone else having this problem?