Nice to see you taking the plunge
Looks indeed like the datasheet of the DSP (it's not the codec) is under lock and key for some reason. Possibly another point where you can find the I2S lines is right where the DAC board is. The ribbon cable should carry all the needed signals. With a bit of luck, you can just disconnect it and make some new PCB with the same connector to harvest the signals from. This would be neet solution for such a board (if you need only 6 channels):
- Multichannel I2S connector -> ADAU1701
- uC to listen to I2C interfaces of volume control chips (I assume that they are I2C and with a bit of luck it's on the same connector)
- uC connected to ADAU1701 to control volume based on what the AVR send to it
- ADAU1701 to 3x SPDIF
If one were to adopt various connectors on there (and support for multiple I2C volume control chips), one could make a kit to retrofit various AVRs this way. Might be a nice project