Amphibole's dashboard

Hello,
In HTML header I have :
soundManager.onload = function() {
first_Sound = soundManager.createSound(
{
id : "first_Sound",
volume: 100,
url : "sounds/test1.mp3"
});
}
Then after I use the following code :
first_Sound.mute();
but it doesn't work...
if I put soundManager.mute('first_Sound'); it doesn't work too...
The only thing working is soundManager.mute(); but i don't want all sounds to be muted
Any ideas ?
Thx