If you have connected your phone by USB it automatically turns off bluetooth audio, and you have reselect it again in order for bluetooth audio to work. This is assuming when you look at the paired phone in the idrive settings the telephone is colored, and the music note is greyed out.
Click on the idrive controller, then click the options button next to the controller, and you'll see the bluetooth options for the phone. The telephone option should have a check mark, and the music will not. Click the check mark next to music and it should work.
You will have to do this anytime you have previously had the phone connected to the car by USB as that disables bluetooth streaming.
This is the problem I had and just figured out last night with help from my service advisor.
Current: '13 E92 M3 Euro Delivery 9/4
Wife's: E39 M5, E36 M3, E30 M3
Gone: E86 M Coupe