MP3s add properly in our tests here. That could indicate some subtle issue with the MP3s on your end, or the QuickTime components installed there.
Can you reproduce this by trying to add a single MP3 file to your Project? If the file doesn't appear in the Chapters list we'd appreciate it if you would send that MP3 to
support@splasm.com, along with a screenshot or complete list of the contents of your /Library/QuickTime/ folder. That way we can try to duplicate the issue here and let you know our findings.