Here's the response I got back from the expert.
About "0 = use sampling frequency from I2S stream", is this not "I2S stream" but SPDIF?
This should probably read
0 = use sampling frequency from audio stream.
This would account for either SPDIF or i2s.
Register 0x15 sets the sample rate when 0x0C[7] = 1.
so when 0x0C[7] = 0 the sampling rate is automatically updated