UdoxMP3Player is a project mainly written in ActionScript, it's free.
Flash (AS3) generic mp3 player, loads XSPF playlists.
This mp3 player can load playlists in the XSPF format
The organisation of the code is such that there is an MP3Player class (gratefully ripped off and tweaked from unformatt.com !!) that exposes a nice simple API and broadcasts useful events that you can build your interface off of.
The rest of the 'view' part of the code is pretty much just an example and you'd switch it out. Shows how to display a playlist, progress bar, volume control etc.
The current UI has a dependency on the OpenPyro component library (which is a very nice pure-AS3 package) which is just used here for rendering the playlist.