I don't think this has anything at all to do with PHP, and more with you needing to make your flash file either read a PHP file that gave it the URL of an mp3 to play, or made your flash file read a GET request or something.

or, if you can somehow use the flash file as a "template," if you will, and then open it during the file upload, and using PHP and edit some variable in your flash file that points to the MP3's permanent URL (and I have no clue if this is even remotely possible, but yeah), and then save the flash file as that person's username or whatever. this might be undesirable though because after a while you'd have tons of flash files, if you have a lot of site users.