Keep all of the video and audio files in one directory.
Then use an htaccess to deny access to that directory from everyone, except, the IP of the site.
That way, only when people request it via a script, will the request come from the site, and so be approved.
Its just an idea though, I'm not guaranteeing that it'll work.
.htaccess
<Limit GET POST>
order deny,allow
deny from all
allow from xxx.xxx.xxx.xxx
</Limit>
where xxx.xxx.xxx.xxx is the sites IP (which can probably be gotten from phpinfo())
Hope that works.