I think each stage will have its own "native tracks", as the pictures showed that Bridge of Eldin had all Zelda tracks, and Delfino Plaza's were all Mario tracks, and that those would be the default rotation for that stage's BGM. However, I think you could probably choose to have it cycle through songs from other stages as well, if you choose. Of course, this is all guesswork.
Minor point, interesting that the "Play" caption over the selected track in the pictures is of the Cube A-button, which definately shows he's considering that the default at this point, and the extra Wii Remote options tertiary. Not that I mind at all.