The first answer will be better than mine, but I notice that when I put a Flash animation on the Master slide it plays continually (and in my case, because I'm puttin a special animation into the background, it was important that the animation not start over and replay with each slide change, but plays uninterrupted throughout the presentation).
I know that if you put your music into previously mentinoed Flash file, it too will play in the background uninterupted and continuously.