Kudos to Marty O'Donnell of Total Audio, who offered some clarification about how Halo would be storing and handling its music. A snip from the forum post:

The idea of switching disks (Riven) is anathema. Not to worry grasshopper, all these issues will be handled wonderfully, and elegantly.

Raises as many questions as it answers, but hey, c'est la vie. ;)