I don't believe HTML exposes any way to adjust this. A lot of the sites you
run into that have a background sound running and some sort of mute or
volume control are actually playing the sound as part of a flash animation,
and the volume/mute controls are within that animation.
The user can use the controls on their multimedia keyboard, or the Windows
Volume Control applet or the controls of their speakers....no need for
anything else.
But what you're referring to is usually built into the plug-in like Windows
Media Player etc, not in the <BGSound> tag.