Looks like this is more of a problem than I had thought. You are the third owner (myself included) that I know about with this problem. It has only happened once to me, so far.
Basically the symptoms are:
1) No audio comes out of the speakers except for the phone. It doesn't matter what audio source is chosen.
2) All of the steering wheel buttons are non-functional. Even the gap button for the cruise control doesn't work.
To bypass this problem, my dealer recommended removing the postive terminal from the battery for two minutes. You'll need a wrench. This will reset the nav/radio but will not loose your settings.
Do you remember anything unusual before this happened? My dealer believes this may be caused by having a CD-R in the CD changer. I believe the actual cause is due to a "No Fob Found" message at the same time as the radio is turning off. So, anything you remember that happened before you noticed the symptoms would be helpful.
I had to have the dealer reset my audio system since the steering wheel audio controls and CD changer went inoperable last week - It did not effect the speakers - now I notice that when using the SW controls to switch channels on the XM they do not automatically switch to XM 2 from XM 1 so u can store and easily use a full 12 stations. Will have to check with the dealer -- also when I switch channels using the XM radio I get sort of a pop/static noise -- Do any of u experience that also?
ctsvman, I tried to PM you, but basically what I believe happens when the battery plus terminal is pulled is that the nav/audio system resets. This probably also resets the GM Lan and class II busses to reset as well, which allows the steering wheel controls to work again.
Perhaps another reset will cause your systems to be restored? In any case, if the audio and steering wheel controls are hanging, this is something that the dealer should see so the dealer can inform the Cadillac tech support people.