I would go along with your reasoning, @morestar, but only the basis the user having already followed a call to action, such as opening a gallery, or something, and then have the music begin ONLY IF the page IS opened from a call to action, not from a external referral or bookmark.
// page 1
$userInvoked = $linkURI$ . "&ui=1;
$callToAction = "<a href=\"$userInvoked\">Start the Slide Show</a>";
// page 2
$soundOn = (isset($_REQUEST['ui']));
As a fail safe one could also check the referrer to see if the request came from onsite or off and set the $soundOn value accordingly.
From a rights standpoint, nothing says fair use like using your own musical content. It's cheesy to hear some standard pop tune and not see a 'Used by permission' disclaimer. We can only conclude copyright infringement in those instances.