Try to low the volume level on your phone. If that does not help then you can try to test different bluetooth audio codecs - setting is available in developer settings of your Android phone.
The vol level stays the same. No matter if max or medium. Only min causes the vol to be muted...
In developersetings there is no point to choose the codec. Running 7.1.1 rooted.
Edit:
At least i found an option in developer settings to turn off max vol at BT in 7.1.1. So now i can control volume of BT from phone. I bit under max solves the cracks.
I also tried it with my friends new mate 20 pro at 9.0. BT codec options are listed in dev settings but can't be changed. All shown options and codecs are greyed out except mono/stereo.