change line 195 of SFEMP3ShieldConfig.h from
#define USE_MP3_REFILL_MEANS USE_MP3_INTx
to
#define USE_MP3_REFILL_MEANS USE_MP3_Polled
This changes the driver not to use Interrupts, but rather the main loop() will
MP3player.available();
will do the refill. This however, requires the main loop() not to be blocking.