Hi,
I don't know if you have already found the solution yourself, but if not I can tell you that the option you are looking for is at the telephone menu. Go to the screen where you make the telephone calls and there should be a settings button somewhere, where you define if during a call the conversation comes out from the front speakers only or from all the speakers. This option affects the music through Bluetooth as well !!!
I was making a demo of my Kenwood unit to a friend of mine last knight and he noticed that the music from the iphone was coming only from the front speakers. I don't use this funtionality a lot and I never noticed that before. I was so disappointed at the time, but when I got home, I googled around and found that this is the solution !!!! Wow, what a relief ...
Hope I helped.
Regards,
dmanou