Why can I no longer control my music apps with bluetooth peripheral devices (headphones, car controls, etc.)? I recently RMA'd my old 5X due to a fingerprint scanner failure.  With my new phone (still 5X), my bluetooth connected devices will not control my phone's music controls (Google Music and Slacker) like the older 5X did.  Is there an update or something that causes this to be opt-in functionality?  Did I accidentally turn it off?  I have no idea, it just used to work previously.  Any help would be greatly appreciated!

I have paired them, and they play music just fine - they just don't respond to skip/play/pause requests - though it does let my devices change volume.  The display in my car also does not show the current track

The devices that used to work and no longer do are my 2013 Range Rover Evoque, Pyle PWPBT60 bluetooth speaker, and Bluez 2 S headphones.
You have to pair your Bluetooth devices with your new phone. Your Bluetooth devices are only expecting to talk to your old phone, which you no longer have. 

See Google's instructions for Bluetooth:


  
  Open your phone or tablet's Settings app Settings.
  Under "Wireless &amp; networks," tap Bluetooth.
  Make sure Bluetooth is turned on and your device is set to visible.
  Your device will scan for and display all available Bluetooth devices in range under "Available devices." If your Bluetooth device isn't showing in the list, tap more More and then Refresh.
  Tap the name of the Bluetooth device you want to pair with your phone or tablet.
  Follow the on-screen steps.
  
  
  If the pairing is successful, your phone or tablet connects to the device.
  If you're asked to enter a passcode, try entering 0000 or 1234 (the most common passcodes), or see the documentation that came with your Bluetooth device.