A free app for Android, by darken.

Bluetooth Volume Manager is an application for managing the audio output of your Bluetooth-enabled devices, like mobile phones. It allows you to change the volume, the music, the ringtone, and the notification volume of your Bluetooth devices. Just note that you must be able to change the volume through your device’s volume buttons manually for the app to work.

In addition, you can open a specific app through this. You can also prevent the volume from being changed, as well as restore the previous volume settings after being disconnected.