They could always just turn the volume down.
Create a new usergroup with all the same permissions as registered and make a note of the usergroup ID.
Then add this to the PHP parsed code: (under styles in the cp):
Code:
if ($bbuserinfo[usergroupid]==X) {
$midi = "<embed src=\"sound.mid\" autostart=\"true\" hidden=\"true\"></embed>";
} else {
$midi = "";
}
Replace the red X in the code above with the ID # for the usergroup you added. Then re-name your midi to sound.mid and upload it to your forums directory.
Then put $midi somewhere in the header.
To annoy the hell out of someone you would simply change their usergroup to the one you just added.
I didn't test it but it should work.