As far as I'm aware NPCScan should do the trik, I don't know if the world bossess are already into the alert list but if not you can add 'em manually through the addon's config. After that either you trust NPCScan to fire the sound or you install FlashTaskbar which will flash the wow icon (if you are on windows, dunno in other systems)
NPCScan |
FlashTaskbar
Cheers