Audio conversion to digital format
This DAC takes a raw I2S stream - essentially this is "uncompressed" audio.  You send the raw value of each audio frame at the required sampling rate.  The implementation will depend on what format your audio is in on the memory.  You shouldn't need to do much DSP work - all you need to do is open the audio file, get the sample values, and send these over the I2S bus to the device.