Securely loading external files

I have an already built flash mp3 player. It works fine, but the client wants the mp3’s inaccessible to to users except through the player.

We currently have the mp3’s themselves in folders inaccessible via http using .htaccess. We bypass this by loading the mp3’s via a PHP script (using file_get_contents()). However, if a user accesses the path of the php script, they will be prompted to download the mp3 anyway.

Does anyone know the method in which sites like myspace securely loads mp3’s? Any suggestions are appreciated.