here is how and see if works for you

Go to CD/Multimedia on iDrive, and select Bluetooth (audio). When the blootooth screen comes up hit the option button, then scroll to Configure Phone and select this. You will see two options, telephone and audio. Most likely only telephone is checked. You will want to check audio, and possibly uncheck telephone since you are not using your iPhone as a phone device.

See if it works for you.