Using MP3BookHelper, I was able to get my files sorted correctly. Although the file name has the track number as a prefix, the ID tag only has the name of the song. By prefixing the song titles in the ID tags with the track number, the files are now sorted correctly.
I believe the dns-323 does not use the track number in the ID tags for sorting.
PS: I am not sure which version of the ID tags need to be updated, so I did them all.