I just thought of something. It's theoretical, since I have never tried anything like what you are describing.
There have been sites (granted, in the past) where I've tried to download an audio file, but I was prevented from doing so. I suspect that it was the way the web server was set up. The audio file was an .mp3 file, and the player that was accessing it was using a protocol I'm not familiar with. If you typed the full URL including protocol into a browser, it wouldn't work; if you typed the path using http: or https: it wouldn't work, either. They somehow set it up so that only the player using that protocol could access the file. I don't know how they did it, but you might try Googling for something like that.
Just a thought.