How to transmit calls only (not the rest of the device's audio) to a bluetooth receiver? When my phone is connected via bluetooth in my car, I'd like the Phone app to be the only one to output through the car's sound system, and let the rest going through the phone's speaker.
Right now the whole phone's audio goes through the car's sound system which prevents me from, for instance, listening to music from a USB key (plugged in the car) and listening to Google maps (running on my phone) instructions at the same time.  I have to chose between the bluetooth and the USB media.

Devices infos: 


Phone: Android 5.0.1 on Nexus 4
Receiver: Volvo Sensus Connect (I think)
As far as I know, the sound protocol is A2DP and the phone is HFP, therefore, they are different. It should be possible to connect only one of them to the Bluetooth device (in this case, your car).
In the Bluetooth devices list, you should have a settings icon next to your car's name where you can disable audio. It should be possible to disable one without disabling the other. I'm not sure of that because I have never tested it and the labeling is quite poor in this place.

If the problem is Google Play Music auto-start, it is possible to force Google Play Music to disconnect from the Bluetooth by clicking on the "Cast" icon in the app itself.