Если вы уже создали свои объекты Sound и SoundChannel.
private var mySound:Sound;
private var mySoundChannel:SoundChannel;
function onButtonClick(e:MouseEvent):void {
var pos:Number = 0;
if ( mySoundChannel ) {
pos = mySoundChannel.position;
mySoundChannel.stop();
}
//jump ahead 20 seconds, the play accepts time in ms
mySoundChannel = mySound.play(pos + (20 * 1000));
}