"I've not encounter that name before. I've seen plenty of linear audio codec's of various sorts. Is this of that same genre?"

I'm not sure what you are asking. I2S is simply the four-wire serial port that clocks serial data into an audio DAC or out of an audio ADC. CODECs (poor name choice, I'll agree) are simply parts that have both ADCs and DACs (for record and playback). Virtually all modern audio converters use the I2S interface (four pins beats 28 when dealing with 24-bit-wide data) and they all are sigma-delta types. Again, see data sheets for parts by AKM, Cirrus, TI and Analog Devices.

And they're all linear. I'm not sure why you'd want to use a nonlinear converter ..

-a