Been looking at the same thing. Tried a Garan audio board or Grove audio board sold by Seeedstudio?
http://www.seeedstudio.com/depot/Grove-Serial-MP3-Player-p-1542.html?cPath=38
http://www.seeedstudio.com/depot/Garan-Audio-Module-p-1607.html
In my application, I want a Jpeg viewer too, for album art etc. That's proving a little more tricky to get at low cost....