I have a pair of Bluetooth earbuds and a pair of Bluetooth headphones I use with my Nexus 6P running Android Nougat (7.1.1).
Two weeks into using the earbuds with my phone back when I first got them, the audio volumes separated: during the first two weeks of use, changing the volume with the buttons on the earbuds would directly change the volume of my phone and there was only one volume control (directly changed by both phone and earbuds). Unexplainably, while I was listening one day, the audio volumes separated: my phone's bluetooth audio volume has a range and the earbuds have their own (the buttons on the earbuds now control an imaginary volume slider that changes their volume but not the volume on my phone). Then the same thing happened two weeks into using my headphones: the buttons on the headphones used to control my phone's BT audio slider, but now change the volume without changing the BT audio slider (each has to be at 100% to get maximum sound now).
How can I stop the audio volumes from being separated between my BT earbuds/headphones and my Nexus 6P and why would they have separated in the first place? Basically, how can I make the volume controls on my Bluetooth devices control my phone's BT volume again?