Mute sound after loading swf

how can I mute sound after loading swf using Loader object?

I tried to use the following code after Loader dispatch complete event:

(loader.content as Sprite).soundTransform = new SoundTransform(0);

but it still play the sound in the first frame of my swf file before the sound is mute.

the code works if I move the start of my sound wave to second frame in my swf, but I need to play at the first frame.

Please help, thanx.