Zyl, I recomment rather this:
1) With some plugin (I think NewXStatusNotify or ZeroNotify) you can disable sounds when having particular status (for example DND)
2) In Idle / Away settings (or maybe with plugin Advanced Auto Away or similar) you can change status when you are in fullscreen mode (i.e. watching movie, playing game..)
And I just see that without plugins you can set in Options / Status / Idle: "Become idle if application full screen" + "Disable sounds on idle" which is basically what I wrote above, just without changing status.