Allright, in your html, where you set the parameter of your swf you can simply add a variable and value like this:
this will create a variable named "Sound" in your flash movie, on the main timeline (_root) with the value "Off". So, in your flash, you can do:
If you are using flash player 7, you can use FlashVars wich is the same thing, except it let you pass 65 ko + to flash player unlike the other way.
<param name="FlashVars" value="Sound=Off" />
It does the same thing...