http://wordpress.org/extend/plugins/audio/

I just checked in version 0.2 of my HTML5 Audio plugin for WordPress, “Audio.” When used with Shuffle, you can now associate an Ogg Vorbis file and/or image with an MP3 file which will all load automatically and simply into an HTML5 Audio player on browsers that support it with a fallback to Flash for those browsers that don’t.

Support for Ogg is crucial because Firefox will not play MP3s natively. Unless you view the source of your code or are the developer of your site, none of this information should matter to you – you can continue to use MP3s, and the code will load HTML5 Audio/Flash behind the scenes. For those who want to take advantage of the latest/greatest and shun Flash whenever possible, use Ogg and MP3.

More on “Audio” here.