There is a DVD audio format - DVD-A.
However in either case, CD or DVD, when there are mp3s on there it is not treated as an audio format at all, it's treated as data.
The manual itself says so, since it says that it searches the folders on the discs for files called .mp3.
It certainly would have been possible to allow mp3s on DVDs.
I managed to get a fair my music onto 4 CDs though, with a bit of chopping stuff I probably would have wanted to have avoided anyway.
I do have a question, though, for someone who has used mp3s + CDs:
When you have it in random mode, does it switch between folders?
I currently burned all my mp3s flat, for fear it might not randomise between folders.
I regret this a bit, since I can't find what I want easily enough.