are you making that webpage, or you're just using it (client) ? if you're not the owner of that webpage, then I don't think you can change it.
But If you're making that webpage and you want to add an embedded audio file, then you're probably using the HTML tag
<EMBED>, and you can use its attribute
autostart to make it (or prevent it) start automatically when the page is opened, just put : AUTOSTART=TRUE or AUTOSTART=FALSE
example
<EMBED
SRC="../graphics/sounds/helloo.wav"
AUTOSTART=TRUE
WIDTH=144 HEIGHT=60
>
is that what are you talking about ? (or am I saying stupid things ? :-\ )
can you display the HTML code of that page ?