Tabs' information densities are already too high. You can't fit most pages' entire title in the tab, so dedicating space for another icon to something that bothers 0.0001% of users isn't very pragmatic.

Besides, how is the browser going to know? There are many ways that a page could be making noise: <audio>, <video>, Flash, Java, other random plugins

Also, how often do you have multiple tabs playing sound? I guess if you middle-click a lot of Myspace links all at once...

If you're running Chrome, why not disable all plugins by default and then set it up so that you can click to enable as needed? That's what I do!