Update: It is now possible to control this without custom code via in5 > Interactive Widgets > Audio.
Attach the following code as a .js file in the Resources section of the in5 dialog (how to attach resources):
stopAllMedia = function (targ) {
if(!targ) targ = document;
$(targ).find('audio,video').each(function() {
var media = $(this)[0], player = media.player || media;
if($(this).parents('.page').index() == 0) return false;
try{player.pause(); media.currentTime = 0;}catch(e){}
});
}
This function overwrites the existing stopAllMedia function and allows media on the first page to play continuously.