Little did I know this would be as tricky as it turned out to be. After a few false starts, including some time wasted trying to extract and convert the PCM version (I never did get that working), I finally found a solution that worked perfectly for extracting the 5.1 audio track.
This how-to was the key to my successful extraction. Becuase that how-to is thorough and includes numerous screen shots, I'm not going to try to replicate it here. In the rest of this hint, however, I'll provide an executive summary (in case the original goes away), and some recommendations for tools to split the resulting MP3 file.
Here's the executive summary version of the hint:
- Install Mac the Ripper and ffmpegx (and its dependencies).
- Insert the DVD and start Mac the Ripper. Select the Mode tab, and then Title-Chapter Extraction from the drop-down menu.
- In the secon drop-down, select the "title" you'd like to extract. Typically this is the longest track in the list you'll see.
- Set the SChpt and EChpt to the range of chapters you'd like to extract (I chose all chapters).
- Click the D button, then the (now enabled) Streams button.
- In the sidebar that appears, remove any checkmarks next to video or subtitle streams, and check just one audio stream, if there are more than one.
- Click Go to extract the audio.
- Drag and drop your AC3 file onto the 'Drop file here' area in ffpmegx. In the Target Format section of the interface, click the arrow in the To box.
- Choose Audio to MP3 from the pop-up menu.
- Select the Audio tab and set a bit rate for the MP3 file, then click Encode.
QuickTime can handle AC3 files, too, so you could probably skip the ffmpegx step if you've got QuickTime Pro (but I didn't test this method). Feel free to share any simpler methods of doing this, if you've got one!

