Based on your comments above, it sounds like you're testing Yahoo Media Player on a Mac by double clicking the .html file, resulting in a URL like
file:///path/to/your/file.html, or using the built-in Ruby on Rails test server.
Having tried it myself, that doesn't work. Yahoo Media Player only loads correctly when the URL in your browser's address bar starts with
http:// and when the audio/video file is hosted on a remote server.
I suggest that you either:
Upload your html/RoR app and audio file to a remote server and adjust the mp3 path accordingly;
-- or --
Upload the audio file to a remote server (it will not work if stored locally), update the audio file's path, and browse to your local RoR test URL to try it. (If you're not using the Rails test server, you'll need to host the html file locally in your
/~Sites/ directory, turn Web Sharing on, and browse to 'http://localhost/~User/test.html` to test it.)