Can you describe the use case more in detail? I ask because I have a similar issue with an Android phone running Android 8.0. I run Spotify in the phone and the phone is connected via Bluetooth to the car. In the phone in the Spotify app it looks like the music is playing, but there is no sound from the car.
Sure so I have a iPhone 7plus running IOS11. When connecting my phone through the cable I am able to get to Spotify which will be playing but no sound coming through the car. My car is a 2013 3 series.