1) I2S is supported only in Master mode as transmitter
2) Have you tested the USBAudioclass firmware in the SDK. What is the delay between the SSN assertion that you see and how much least can you work with?
The delay between SSN assertion is approximately 4-5 clock cycles. We've been looking for some type of SPI audio bridge that can input the I2S from microphones but we've found nothing so far that doesn't require a constant SPI clock.
Just in case anyone else has this question, tech support responded that there isn't an off the shelf component that does this in such a way that is compatible with the spi lines as controlled by the CX3. Instead, they recommended using a small Lattice FPGA that takes in the audio data and then can buffer it in such a way as that it can be read by the CX3 over the spi bus.