What are you using to play the audio on the client side?
Flash-based media players such as
FlowPlayer can be fully manipulated using JavaScript. This means you don't need to include the full path to the mp3 file anywhere in your web page. You can include some bits and pieces of the path in the document, assemble the rest of it using JavaScript, and pass the result to the Flash-based player. (For example, you can store "dQw4w9WgXcQ" in some variable, and use it to assemble the
full path.) Most browser plugins won't be able to detect that.
FlowPlayer also has a
secure streaming plugin. I haven't used it, but it might provide some sort of DRM facility.
But of course, none of this will prevent a savvy user from ripping your audio.