This was very illuminating:
Playing MPEG stream from SOU2003DemResponseS192SS0.m1a ...
MPEG 1.0 layer II, 192 kbit/s, 32000 Hz stereo
Thus it looks like I need to transcode that audio.
The next trick is going to be figuring out how to master a DVD image with chapters. You can create multiple titles with Sizzle right now, but that's different from chapters. Ultimately, I want to be able to mark the locations of commercial breaks (which hopefully I can remove) as chapters.