Which PIC MCU for an Audio player?
The dsPIC seems to be not powerful enought to play an mp3, although you can use a dedicated chip to decode and the dsPIC to play the music and read the file on the SD card. A tutorial on a similar project can be seen at http://www.uchobby.com/index.php/2008/07/21/dspic-wav-player/ 

Microchip themselves have an application note on how to port Helix MP3 decoder to PIC32, (AN1367: http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&amp;nodeId=1824&amp;appnote=en551513) so that may be the natural choice.