Some further investigations: I have found this code from the I2S library. The I2S lib is the base of Arduino's Sound library a obligatory prerequisite:
https://www.arduino.cc/en/Tutorial/I2SInputSerialPlotter:
This example shows you how to read and visualize on the serial plotter I2S audio data coming from an I2S microphone.
I wonder if I do not need the sound library, but only the I2S lib and write al incomming data (to serial in the example above) to a SD card.
What kind of data is this? How can I make a wav or better mp3 or ogg from this stream. Is this possible with a Cortex M0 board?