For putting to work together the Ethernet shield and the Sparkfun MP3 shield with the Arduino Due, I make the following modifications to the shields :
First, I use the Ethernet shield to transmit to the MP3 shield the SPI signals, soldering 3 wires between the SPI connector and the digital pins 11, 12 and 13. I bend these pins to isolate them from Arduino Due. Photos at http://imageshack.us/photo/my-images/585/58497352.jpg/ and http://imageshack.us/photo/my-images/96/16410217.jpg/
Second, I bend pin 4 off Ethernet shield and I cut the bridge between digital pin 4 and GPIO1 on the MP3 shield. Now I can connect chip select of SD card of Ethernet shield to the 3V3 so I disable this SD card.
Third, I bend pin 6 of MP3 shield so I can redirect chip select of MP3 chip to pin 4 of Arduino Due, allowing the use of extended SPI functions. Photo at http://imageshack.us/photo/my-images/801/34017413.jpg/
For the software, I modify the library written by Bill Porter and Michael Flaga. I add an example WebRadio.
It works fine with most radios than transmit up to 64 kb/s
The sketch prints some statistics every seconds. If I don't make any mistake I can say thanks to this stats that the limit is due to the W5100 chip of Ethernet shield.
Somebody can confirm that it is not possible to obtain rate higher than 80 kb/s with W5100 chip ?
My fork of SFEMP3Shield is at GitHub - gallegojm/Sparkfun-MP3-Player-Shield-Arduino-Library at Due