This app gives your Android device the ability to have individual music and voice volumes for each of your bluetooth devices.

It remembers the volume settings for each device and restores them if necessary.
Volumes can be configured through the apps UI or the volume buttons your device.

This app was motivated by me trying to listen to Spotify through a car radio.
Android 7.X+ keeps resetting the bluetooth volume to 50% everytime a device (e.g. headset, speaker, car stereo) connects and the volume was above 50%.
Official ticket: https://code.google.com/p/android/issues/detail?id=212402

Permissions explained:
• 'Internet' for bug reports.
• 'Bluetooth' to work with bluetooth devices.
• 'Modify audio settings' to change volume.