The file that the media player is looking for is:
overton.sundayschooldesign.com/media/images/audio/17thaug08.mp3
Does this file exist? You can see what it's looking for by examing the source of your page.
When I type that url into my browser I get a file not found.
Take a look at how you have the servers/folders and then filename entered. You are building a url by these three things (or you can just put the whole thing into the filename field under mediafiles). The component adds the http:// if needed.
Hope this helps. Let me know if I can help further.