You need to enable the Bluetooth Audio Player in the MMI settings. You also need to dig deeper in the MMI menu to find the Bluetooth connection setting and enable media audio for the paired BT device (presumably a smartphone), by default only the call part (phone audio, address book and such) is enabled. Make sure the media audio is also enabled in your BT device's connection setting.
It even carries the YT titles you're playing onto the MMI unit. Of course Audi don't want you to know that so they never advertised the function, but it's there and works decently good. Just take your time and read the User Manual, it's described there.