A few ideas..
1. When initially pairing your phone with the car, it gives you the option to pair only for phone calls or to pair for calls + music. Make sure you enable both.
2. Try removing your phone profile and re-adding. Try playing songs that are saved on your phone (i.e. not from youtube/spotify). Keep in mind that you probably have to crank up the volume a good amount to hear stuff through your phone). Does the car actually say your phone is connected or are you just selecting the iphone you're trying to connect to? When playing music from your phone, does your phone say the output is to the bluetooth connection?
3. Is the idrive up to date? You can check here:
https://www.bmw.com/en/footer/software-updates.html