I have HK sound and when there's voice guidance the music volume automatically lowers during the instruction.
Was like that from day one. So I don't know if theres any settings for it.
EDIT: To be a bit more specific thats with iPod, I've not actually listened to the radio while using guidance.
330D M Sport Estoril Blue 19" + M Sport Brakes, Adaptive Suspension, HK Sound, HUD, Stop+Go, Vis Pack, New Pro Nav.