Is it possible to attach an mp3 to an auto-response email?

Ideally, we would like to send this mp3 download to people's emails who sign up using this special form we've created so that only these people receive the file and it isn't hosted online for all to access. Right now, I don't see it letting us attach an mp3 to an auto response email.

It is not currently possible to attach files directly to the email.  It would need to be hosted elsewhere like Dropbox or Google Drive.  You could then include a link to the download in the body of your email.