I think you must read the AS3 doc: http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/media/Sound.html
As you can see you just have to create a new Sound object:
var mySound:Sound = new Sound( new URLRequest('http://url.to.your.sound') );
or if you want to wait for the load complete:
// create sound object
var mySound:Sound = new Sound();
// listen for the end of the loading
mySound.addEventListener( Event.COMPLETE, _onSoundLoaded );
// maybe you want to track the progress of the loading
mySound.addEventListener( Event.COMPLETE, _onSoundLoading );
// start loading
mySound.load( new URLRequest('http://url.to.your.sound') );
// the loading is complete
function _onSoundLoaded(e:Event):void
{
// play the sound
mySound.play();
}
// the loading progress
function _onSoundLoading(e:Event):void
{
// trace the progression
trace( mySound.bytesTotal / mySound.bytesLoaded );
}
Hope this helps.