Sandy Macdonald, one of the Pimoroni crew, has written an excellent tutorial in which he takes a pHAT DAC audio board, a Scroll pHAT and a Raspberry Pi Zero, solders them together and creates a visual spectrum analyzer. Python is used to analyze the audio being played via the DAC and then pump the readings out to the Scroll pHAT. Read how he did it here.

You can get hold of the pHAT DAC from Pimoroni or The Pi Hut, and of course you can also get the Scroll pHAT from the same places: Pimoroni, The Pi Hut.