Analysis of information sources in references of the Wikipedia article "I²S" in English language version.
Clock source for the delta-sigma modulators and digital filters. ... It is the clock that is used by the audio codec ... to time and/or drive its own internal operation.
True, the master (modulator) clock is not part of the I2S standard
Audio converters based on the delta-sigma (ΔΣ) architecture require an internal master clock that operates at a much faster rate than the target sample rate.