Probably the best way to go about that would put your audio content in a members-only section of your site, where users would need to log in to access that area, then use our custom/3rd-party Integration feature to have each buyer's order data sent to a script that would auto-register them in your membership system:
http://www.e-junkie.com/ej/help.integration.htm
Our Integration submissions follow the format of a PayPal IPN, so any membership script that can accept PayPal IPN should be able to accept our Integration submissions, as long as they can handle or ignore the extra fields we add to the standard IPN data set, and if they don't need to (or can be configured not to) verify the submissions with PayPal, which would fail since we are not PayPal. This also means you can send "PayPal IPN" to your membership script even if you accepted payment by non-PayPal means (e.g., if you prefer to use Google Checkout, Authorize.Net, etc.)
For a somewhat simpler approach than that, after payment is completed you could simply redirect buyers to a secret membership registration page on your site, where they'd fill out the registration manually. In this case, you'd still want some custom scripting to verify that users are arriving at that page via our Redireciton link, so buyers can't just share the URL of that landing page with others who could visit it to register without paying first. This thread discusses some approaches you could use for that:
http://www.e-junkie.com/bb/topic/5205#post17645
