Not sure how sirius works, since I have XM, but I have a feeling they are relatively the same. Jinxxy....does the HU say it is sirius ready? I know my previous HU said it was XM ready, only to find when I was getting the install done that you have to buy an additional $150 part to "tune" into the channels, and then that unit plugs directly into the HU. I just said screw it and bought the newest Alpine HU that has everything built right in so all I needed to do was hook up an antenna and BOOM....XM radio. Your manual will tell you what you need (if you need anything in addition to what came standard, if not contact Audi.....after all.....it's their install, and equipment, right?
Sean
PS.....for anyone wondering....XM is just as "muffled" with the volume as anything you are all talking about. Not sure it was worth it (since I'm a CD listening nut....I hate radio!!!! But my gf got me the XM for XMAS, so I'll try it out......)