You can do this by using a server backend such as Parse. They have a free SDK. There are other similar options and I believe apple is rolling out the same thing in iOS 8 with CloudKit.

Absolutely. There are different ways of accomplishing this there it depends on your current setup.
Apple likes to have one inapp purchase element for each content but you cans till get away with simply switching out the actual content that gets loaded and "re-using" the inapp.
Let me know if you want to further discuss, I've been dealing with inapp annoyances and tricks for a long time.

Yes, of course - it can be easily done. One way would be to have the user purchase the in-app purchase item, check for confirmation and then start downloading the audio track from your server. Let me know if you would like to discuss.