found this off to try at lunch

Hi DbloX5,
I had the same issue including SatNav working but no sound, took X5 to my BMW mech. He turned the ignition off, removed and reinstalled the #7 fuse in glovebox, checked radio. Still didn't work, so then he turned ignition off again and opened up fuse box in cargo bay and removed and reinstalled the #72 & #73 fuses and turned radio on and it worked. No probs, he told me that when the battery gets low then it starts sending items into sleep mode to protect major components as noted earlier by pmelche. He also said another problem is if water gets into the radio control module, it wil blow and then needs to be replaced. The radio control module is near the battery under the spare wheel.
Hope this helps.
ptr_bkr.