I'm trying to add on my site new feature. I want people to download audio files but files must be protected somehow (password?). Only one person (account owner) could listen to this file. Is is even possible?
Yes it is. For this make for example a file called audio.php in this one you check the user permission and then set the headers to send an audio file and output the audio file afterwards, if the permission is not approved show an access denied.
How the audio headers are done is discribed here: Output mp3 with php