Recently I had an issue with Anker Soundcore Mini Bluetooth speaker. Windows volume level was not working with it. Only volume level 0 worked. Any other level just unmuted the speaker.
When changing the volume from the anker it worked and volume on Windows was also changed.
Here is how I was able to fix it:
Side effect is that the volume control on the anker does not synchronize with Windows.
Credits: http://community.anker.com/t/cant-change-volume-via-windows/66148/4