The Rasperry Pico Core implementation provided by Arduino is based on MBED and does not have any I2S implementation. The core from Earle Phil Hower however has one which relies on the pico-extras from RaspberryPI.
I am combining these two solutions as a separate Arduino Library, so that it can be used with the Pico-Arduino and the official Arduino RP2040 board setting.
Here is the class documentation
You can download the library as zip and call include Library -> zip library. Or you can git clone this project into the Arduino libraries folder e.g. with
cd ~/Documents/Arduino/libraries
git clone pschatzmann/rp2040-i2s.git