Amazon didn't advertise music or audiobooks being part of FreeTime, but you're also forgetting about this being for the smaller kids who would obviously have a hard time navigating something like the Audible app (I hope this is leading to developers being able to take advantage of pushing out stuff to the carousel), and who would be using the enhanced KF8 anyway (well at least Amazon is pushing for this idea
).
When you're watching video, and playing a game that already has a lot of sound effects, music obviously isn't needed anyway, it's just a distraction. I think the whole point is to NOT just leave your kids there to look at pictures, but to efficiently gain knowledge/skills in that time limit that's set.
They might not have good developers, but they did really well in making it really, really hard for someone to get out of that mode that shouldn't be!