Yes, I suspect the Audio-20 (option 523) would have been removed when 512 was added as you can't have both - and although i've never seen a car without Audio-20/Audio-50 or COMAND (because UK spec always gets Audio-20) it makes sense that without option 523 the audio menu would get turned off as the 'MB system' would beleive there is no valid audio system (523/525/527) in the car.

Cheers
Richard