Is it idrive or pro radio just to be sure.
First determine which sound system you have and I don't
mean which head unit. (stereo , hi fi or logic 7)
Next check fuses if the rest of the car is not showing signs of
a low battery. (Date and time lost ) I would put a slow long charge on the battery just to be sure. Measure the battery volltage resting and the alternator
with the car running. battery above 12.4 alternator 13.5 to 14.5
If the car has been sitting a long time chances are it has killed the battery.
The battery has to be on a battery tender when in storage or disconnected.
The IBS system will turn of non essential modules such as the radio if
the charge is to low. read about IBS in the first pdf.
electical e90 pdf
http://www.google.com/url?sa=t&rct=j...,d.cGU&cad=rja
http://www.google.com/url?sa=t&rct=j...85464276,d.cGU
http://www.google.com/url?sa=t&rct=j...85464276,d.cGU