Hmmm... not sure about doing that without designing the entire site in flash (though i am by no means an expert, and have only tinkered with flash). The problem is that everytime you open the index.htm page from another page (volunteers for example) it is the same thing (as far as the browser is concerned) as refreshing the page.
Since your flash is coded to play upon opening index.htm, that means anytime you open it by any means (whether refresh or link) it will play that file.
What you could do is create a page with the header static, and then link to that from all your other pages. (e.g. index.htm takes you to volunteer.htm, but if you click 'home' from there it takes you to index1.htm which doesn't have the swf file on it).
Just a thought.
