I have a Sept 05 325 with ProNav. I have it booked in to upgrade to CIM20 tomorrow - I will post whether it plays mp3s.
I am not sure that the cim20 will fix your sony connection. I had the same problem with a k700i and couldnt get it working - connects, disconnects. I also tried a w800 and it did the same thing. I ended up acquiring a friends old siemens S65 and it works like a charm - just a year or so older.
325i, Black Sapphire, 6sp Man, 18” Ellipsoid, Black Dakota Leather/Aluminium, Sunroof, PDC Front/Rear, Electric Seats, Professional Navigation/iDrive/TV, 6 CD Stacker, Oct 2005