Out of curiosity, I just dropped the example code from the Arduino for the Due Audio library
here straight onto the DigiX, and aside from changing the filename, it worked!
Admittedly, the first time it sounded like the Chipmunks were singing, but adjusting the sampling rate in the code from 88200 to 44100 cured that. I took absolutely no care with generating the WAV file, I just dropped an MP3 into Audacity and saved it as a WAV file using all the default settings!
When I've got more time, I may see how much effort would be involved in getting an MP3 software decoder onto the DigiX, but I suspect it's a fairly non-trivial amount of work to extract the necessary bits of code from something like the LAME library.