I've been wracking my brain for a good way to do the transitioning, @magic, but I haven't come up with anything.
This is a scenario that would benefit from an automatic transition of mockups - something that has been requested by others in the past. I will check in and see where we are with that, and if it's something we want to talk about again.
As for adding music - there isn't a way to do that. I'm sorry. You could annotate your mockup with icons or markup to say "this will be spoken", but you wouldn't be able to add a track to the mockup itself.
I'm sorry that I don't have better news for you today, Leslie, but don't let this discourage you when it comes to feature requests. Seeing that folks need stuff like this really helps us prioritize changes, and makes the app better as a whole. If there is anything else you'd like to see, let me know, and we can talk about it!
If we don't talk beforehand, have a happy weekend.