Actually simpler than all that above. Change line 145 of SFEMP3ShieldConfig.h from
#define MP3_DREQINT 0 //Corresponding INTx for DREQ pin
to
#define MP3_DREQINT 1 //Corresponding INTx for DREQ pin
This will swap the interrupts and should match the Leonardo.
If that works I can put in a Flag to auto select the correct value based on board type.
