I read a thread once where the aux output volume had a separate control, 1-6, w/ 6 being the loudest, and people would occasionally find this volume reset to 1. When this happened, their aux output (i.e. iPods, etc.) volume was quite low. There was a series of buttons to push to get access to control this voluem. This was on e46's, but perhaps your situation is something like it. Search threads on e46fanatics (think they have an e90 section?) or bimmerfest, they've got a lot of audio experts, you might find a thread.