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.

https://www.raspberrypi.org/forums/viewtopic.php?f=45&t=57069

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.

http://www.ti.com/product/pcm1863/

Same for this board here.

http://www.ebay.de/itm/UDA1380-Board-Stereo-MD-CD-Mp3-Audio-Decoding-Decoder-Module-Development-Kit-AL-/272160416012?hash=item3f5e06610c:g:MnYAAOSwP~tW3i8q