Shield incompatibility error

I am using the Sparkfun Mp3 Player shield and a set of speakers. In addition to this shield, I am using the Shifty VU Shield to read the audio levels so that I can synchronize some LEDs with the audio decibel levels.
Seperately, the two shields work perfectly. The Mp3 player shield, connected to the Arduino ONLY, can play the music through the speakers I've connected. The Shifty VU shield, connected to the Arduino ONLY, reads the audio levels sent by my phone via AUX cable while playing pandora.

The problem is that whenever I stack the two, the music simply stops working. I can't, for the life of me, figure out why it would do that. Everything is connected EXACTLY as they would be seperately, just stacked on top of eachother. Without even connecting the two through AUX, just stacking them, and the music stops playing.

Any ideas why?

Compare the pins used by each one. It sounds like there is a control signal conflict.

Yep, looks like they both use pin 9, 11, 12 and 13

The Shifty VU Shield I think only uses the SPI pins if I connect something to it. I literally am just stacking them to see if the MP3 player shield will play sound. The shifty VU is only getting power, nothing else.

millencolin543:
The Shifty VU Shield I think only uses the SPI pins if I connect something to it. I literally am just stacking them to see if the MP3 player shield will play sound. The shifty VU is only getting power, nothing else.

But by being powered and connected to the same pins it is probably causing the problem.

Whats can I do about this? Because they have worked together before in the past. I'm not sure why they are acting up now...

Anything?

millencolin543:
Anything?

OK, first look closely at the hardware. Find the schematic drawings of both boards and look and see what shield pin numbers they do use and see if there is a conflict. Or provide links for us to the drawings of the board and let us have a look.

Then post the codes. As you say both boards work fine separately but not together, is that with different sketches for each board or the same sketch testing with each board separately and then together?

Lefty

http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Dev/Arduino/Shields/MP3%20Shield-v13.pdf <-----Sparkfun mp3 player shield

http://docs.macetech.com/doku.php/shifty_vu_shield <---------- Shifty VU shield

Yes, they are using the exact same code.
Thank you for helping

millencolin543:
they have worked together before in the past. I'm not sure why they are acting up now...

Previously you wrote: "whenever I stack the two, the music simply stops working". That doesn't suggest they have ever worked together. Which is it?

It is both. About a week ago they worked together fine, but I was using headphones for the music. Then I installed the speakers and the mp3 player shield played music out of it just fine, UNLESS I added the shifty vu shield for some reason. After I installed the speakers, the shifty vu shield became incompatible with the mp3 player shield. But right now that is not my biggest concern anymore. What is a concern is that I am using a C++ program to continuously send data to the Arduino and whenever this program is running, the music (with only the mp3 player shield attached) has crazy electrical interference or something. When this program isn't sending data to the serial port, there is no problem at all and the music works fine. Not sure what this could mean.

The first problem might be a power supply issue.

As for the second problem, I don't know how your MP3 shield works but does it make any use of pins 0 and 1?

Your question implies an answer that could make sense. I am not very experienced with electronics in general, but that is the schematic. The Arduino is receiving data from CPU