I've received a few error reports on a for-fun-only flash thing/experiment/joke I did some time ago, the next captcha generation for myspace forms.
At first I thought it had been my fault, due to my inexperience in flash, but after having been working more time with sounds in flash, I found the problem is not in my code but in Flash's internals. For some reason, if you're calling, for example, computeSpectrum in one movie and at the same time there's another flash movie being executed and playing any type of media in another tab, your movie will begin to trigger Security sandbox violation errors.
It seems to happen because the player doesn't distinguish between different movies, and treats all the sounds as if they belonged to the same soundspace (like a namespace, but for sounds :D), hence it tries to mix them all together and process the soundSpectrum of this global mix. Obviously, they don't belong to the same domain, and that's why you'll soon get errors.
And you might wonder what's going wrong, because you've got your policy files and everything sorted out, everything seems fine but it's not when you try to access global sound stuff. Hopefully now you know why (although you can't do anything about that).
I'm not sure if this should be an expected behaviour or they just didn't think that we could execute several flash movies in different tabs. Obviously, this only shows up if you're using the debug version of the player. In any other case, all these errors are lost in a flash log file which is hidden somewhere in your hard disk.