I removed the top layers of bricks today to see if I could determine what's happening to my wall. Here's what the wall looked like when I discovered the problem on New Year's Day.
As I pried of the bricks today, I could see that my suspicions were confirmed. The space under the asphalt in front of the wall is completely hollow.
I can actually see the first level of brick, which has slid under the street and toward the source of the sinkhole. Guess it's time to call the city.