user SnickelFritz from Kirupa gave me a fabulous answer with much easier code, which can be very powerful when using multiple sounds. This is just great, because you can use only one preloader per SWF, instead of having multiple loader for each file:
code for the main.as file
http://www.kirupa.com/forum/showthread.php?305098-Playing-a-embedded-sound-in-an-external-swf
package
{
import flash.display.*;
import flash.media.*;
import flash.events.*;
import flash.net.*;
public class Main extends MovieClip
{
var swf:MovieClip;
public function Main()
{
var l:Loader = new Loader();
l.contentLoaderInfo.addEventListener(Event.COMPLETE, swfLoaded);
l.load( new URLRequest("child.swf") );
}
private function swfLoaded(e:Event):void
{
swf = e.target.content as MovieClip;
addChild(swf);
swf.playSound();
}
}
}
Code for the loaded swf "child.as"
package
{
import flash.display.*;
import flash.media.*;
import flash.events.*;
import flash.net.*;
public class Child extends MovieClip
{
private var s:Sound1;
private var sc:SoundChannel;
public function Child()
{
s = new Sound1();
sc = new SoundChannel();
// All of the following lines are actually optional, if all your graphic elements are already inside the swf prepared in flash pro
var g:Graphics = this.graphics;
g.beginFill(0x666666);
g.drawRect(0,0,550,400);
g.endFill();
}
public function playSound():void
{
sc = s.play();
}
}
}
User "kglad.com" on the Adobe Forums also gave me a good answer: