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=212402Permissions explained:
• 'Internet' for bug reports.
• 'Bluetooth' to work with bluetooth devices.
• 'Modify audio settings' to change volume.

// Option A //

To download Bluetooth Volume Manager mod from HappyMod.com.
You need enable the option "Unknown Sources".
1. Click on the above link to download Bluetooth Volume Manager mod APK.
2. Save the file in your device Downloads folder.
3. Now tap on Install and wait for the installation to finish.
4. Once it is done, open the game and start playing it right away.

// Option B //

To download Bluetooth Volume Manager from HappyMod APP, you can follow this:
1. Open your browser and download the HappyMod APK file from HappyMod.com - the only official website of HappyMod.
2. Open Android Settings and go into Privacy or Security.
3. Tap the option to Allow Unknown Sources and enable it.
4. Go to your Android downloads and tap the APK file.
5. Follow the directions on the screen to install it.
6. Search Bluetooth Volume Manager in HappyMod App.