I would simply regard it as a normal product, the audio file! Then during checkout when the order is complete (not submitted but more like pending update order balance .. see other thread about it for details), you email the customer a special url or if it is not taht big dispatch an email with it. You could also thus create a zip file with password so that only this user can download and open it until you have deleted the special file link again ...

If you like this approach, let's talk more about it (I do not know what the uc file download module offers at this point?! Try it out 2