The correct way depends on what you have to work with.
Web browsers are what you call client software. If you use the term 'Web', you are talking about the HTTP protocol, and URLs that start with http://, and your "web browser" is what is called an HTTP client. Thus if you put something on the Web, you then tell other people how to get it by saying 'http://domain/path/file', and they can use their web browser to get it.
But hold on! Another protocol is FTP, and thus you need FTP client software to use it. While Filezilla is an FTP client, you should be aware that most desktop web browsers are both HTTP and FTP clients. Thus on web pages when a link is to an FTP site (ftp:// URLs remember!) your web browser will be able to download the file for you.
As I mentioned these FTP URLs are not suitable for your purposes because you don't have an FTP server that won't require a username and password. So that leaves you with HTTP URLs that link to your web site - there is no requirement for a username and password in order to download a file from the web site. More importantly in your situation you have to take into account whether an FTP or HTTP URL is required when an application asks for a URL. Yeah, you know that Firefox/Opera/Safari/Chrome/MSIE will do both, but did you notice the media player that you want to use doesn't mention which type to use? It could turn out that only HTTP will work and FTP won't. If documentation doesn't mention a particular scheme (ie ftp:// or http://) it is safe to try an HTTP URL first.
As for as making an HTTP URL it's rather easy if you're talking about linking to a file with no frills. You already know it will beginning with http://domain/ and that your home directory is /home/username and the web files are in /home/username/domain/ (called web directory) right? Well your web site works by trying to match the url-path to a file path. The url-path starts with the first slash after your domain, thus for http://example.com/ it is simply /. So to find a file we take /home/username/example.com and prepend that before the first slash, getting /home/username/example.com/. So in your case:
file path: /home/username/oursite.org/Media/MP3/mp3filename.mp3
web directory: /home/username/oursite.org/
HTTP 'root' URL: http://oursite.org/
(now swap web directory for HTTP 'root' URL in file path - ie mapping)
HTTP URL: http://oursite.org/Media/MP3/mp3filename.mp3
Note that I added a trailing slash to the web directory. That is used to indicate it is a directory and not a file. Sometimes it is not needed because it is assumed you are only giving a directory name and never a filename. Usually documentation will point out if a trailing slash is needed or not in a file/directory path. (HTTP url-paths are not necessarily file/directory paths; they get 'mapped' to one if there is no special action to be taken)
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