How to Fade Out a sound loop without using a button?

Hi.

I have a small presenttion that have a Loop of a song starting to play on the Scene 1. The sound plays in loop along the several scenes and I want it to Fade Out on Scene 12. Is there any way to Fade Out without using a button to do that?
I have tryed the Fade Out on “Effects” (on Properties panel), but doesn’t work…