@cr08 Thank you for that information! I now know way more about my car's audio modules than I did before. It would have taken me a long time to do that research on my own. That's exactly what I came to this forum hoping for: someone who already has the expertise and can summarize for me. So, thank you!!!
I decided to have the dealer just do basic maintenance and inspection so I could delay the decision about what to do and how much to spend until January. And when I got the car back, the audio was working again! Apparently, what it needed was just a reset. Now that I think of it, in the past I have reset the modem and various Sync computer functions using the fuse panel under the dash. (I've been so busy lately that I forgot about that.)
Anyway, in January I can get the 60K service done and I will search to see if someone other than the dealer can do that for me. I also found out I need new tires.
Thanks to @Lacienega, too. In silicon valley, everything's expensive. (Gas prices peaked at $9.75 a gallon.)