The idea is to keep these files outside of your site's root directory, because anything within it is accessible by a URL.
So if your site's root is located at /home/[user]/htdocs/ then you would not want to place them inside of the htdocs folder. I would put them in /home/[user]/mp3/ or something similar outside of the htdocs folder. This way, the files are stored in a directory that the web server isn't set up to 'serve', and thus, not accessible by a URL.
Keep in mind, the location of the htdocs folder I'm referring to will vary. It may be something like /home/[user]/htdocs, /var/www/, or /www/html...it all depends on how the system is configured.
BTW - This may not be possible if you're running off a web host...depends on their configuration and your permissions. I know with Dreamhost I'm allowed to configure each of my site's root folder, so I can manage a trick like this. Other web hosts I've dealt with aren't so permissive.