If I want to use the cluster with the alexa service then I need to find an I2S, SPI or I2C soundcard to get microphone input. I can't really use USB soundcards, since all 4 ports are taken by the 4 PI zeros and vice versa.
pi dac+ and hifiberry dac+ don't have any ADCs but in the datasheet of the used chip 5102A there was also the PCM1863 that might be able to help me out. not sure. Cheapest chip is 5-7euros on ebay.
Same for this board here.