My good colleague Max recently released his Yahoo! Media Player WordPress plugin. This add-on uses the Yahoo! Media Player, recently released on the Developer Network, to automatically create an embedded MP3/audio player for every link to an audio file you have in your posts (or every file linked from an XSPF playlist).
Additionally, Max’s plugin makes it easy to toggle between the production release and the more cutting-edge builds.
You can see an example of this plugin in the bottom-left corner of this page… just hover over the grey tab, and a pop-out bar will provide controls for playback of a series of MP3s I’ve hidden in this page’s code (courtesy of hip-hop mixtape master Mick Boogie).