If you just tell it to loop 9999, then that should be enough. No one is going to stay on your menu long enough to listen to it 9999 times but if not you can add a function/listener that restarts the sound when it's done playing.
// AS2
var mySound:Sound = new Sound();
mySound.attachSound("mysound");
mySound.start(0,999);
or in AS3 set up a class in the library for your sound that extends the Sound class.
// AS3
var s:Sound = new MySound();
s.play(0, 9999);