Firefox should be able to block Flash audio just like it blocks pop-ups. Users should be able to unblock the audio just once, or always for the site (again just like with pop-ups). An icon in each tab could indicate the status of the audio, and control if it’s blocked or not.
There is a Firefox extension called Stop Autoplay, although it doesn’t do everything you suggest.
Another option is Flashblock. With that you can control flash just like you do pop-ups, meaning you can block or accept websites. Be careful when you turn the extension on/off because sometimes it crashes.
