I'm using a Creative Zen Vision:M with WMA files through my E90 and the volume is fine for most ripped CDs. Some of the older ones dating back to the late 80s early 90s seem to be significantly quieter but all the latest stuff will go more than loud enough to make my ears sore. A mate of mine has found that ripping his CDs to his iPod produces lower volume recordings than downloading from iTunes so I guess there's some mileage in the volume possibly being configurable - don't know how though as it doesn't seem obvious with the Creative software.
E90 320iSE Sapphire Black + split/fold seat, folding mirrors, sport steering wheel
12 and all complete rubbish - not a BMW among them!