As far as I know, Ektron doesn't provide any player / plugin support ootb for MP3 files. You can specify an additional MEDIA type in the web.config, perhaps. Or use HTML5 Audio. http://www.w3schools.com/html/html5_audio.asp
When using the HTML5 approach, pay attention to which codecs are supported by which browsers. You'll have to work out something else for older browsers via fallback or shim.
Adding a MEDIA type in the web.config will just tell the browser to load a plugin like quicktime or wmp, if it works at all.