MP3 Shield stacked on DMX Shield - Arduino Uno
Yes, overlapping is a problem, by default the MP3 Shield uses digital pins 2(DREQ),6(mp3CS),7(mp3DCS),8(RST),9(SD-CS),11(MOSI),12(MISO),13(SCK)

The Tinkerkit DMX Master Shield R4 uses 2(RE-ENn),3(VTX),4(VRX),5(DE-EN),6(D6)

So there are conflicts with D2 DREQ vs RE-ENn and D6 mp3CS vs D6.

The DMX Master Shield basically converts the UNO's serial to DMX. If nothing is at first included into the Sketch for this, then the problem is with only D6. Leave this jumper OFF. And I suspect the MP3 Shield will work stacked.

To use them together you can move the pins, as demonstrated at https://www.youtube.com/watch?v=behMgV3tUr4. Noting that is for different pins. And of course you will need to subsequently change their definition, in the sketch.