The LED cube
already has an Arduino Leonardo-compatible built in to the main PCB, so all you need to do is build a small circuit that contains the spectrum analyzer which connects to the I/O pins on the cube PCB and also has audio input and output connections. First, consider the schematic in this step.
For the purposes of this project our spectrum analyzer will only display the results from one channel of audio – if you want stereo, you’ll need two! And note that the strobe, reset and DCOUT pins on the MSGEQ7 are labelled with the connections to the cube PCB. Furthermore the pinouts for the MSGEQ7 don’t match the physical reality, so compare them against the pinouts in the second image.
The circuit itself will be quite small and fit on a small amount of stripboard or veroboard. There is plenty of room underneath the cube to fit the circuit if so desired.