iPhone 6 - sound drops to half normal

I have experienced a weird anomaly on my iPhone 6. While using Beats Wireless 2 bluetooth headphones, I sometimes experience a drop in volume while in iTunes to about half of what is normal. The volume slider is at full but the volume drops to half. I couldn't figure out was happening or why. I went in to the Apple store and they gave me a new phone. Same problem.

This does not happen when using my Bose wired headphones.

Here's the weird part. During one of those drops, I heard a beep signaling an e-mail had been received and when I went to open the e-mail, the volume went back to 100% as soon as opened the e-mail. ***? I am still experiencing these drops but now I know how to fix it. I send myself an e-mail and soon as I open it, the volume goes back up!

What possible connection could there be? Why is the volume dropping in the first place with Bluetooth? Why would receiving an e-mail fix it?

Anyone else experienced this?

Thanks.