Hi chuckwin,
It looks like this is an old thread, but I'll try to answer if it's helpful for you.
I personally have fried a few ADAU1701s in my day. I believe there are internal overvoltage protection diodes in all of the IO pins, but there are only designed to protect against short spikes and were not designed to protect long-term abuse. If you overdrive them regularly, they will fail.
The solution is to put TVS diodes on any pins that might see an overvoltage event. I would suggest doing this especially on the ADCs (if you're unsure of what somebody might plug in to your product!) and definitely on the main 3.3 V supply line.
I have had very good results with the NSR0320 from On Semi.
Layout it key. It's quite important to place the device as close as possible to the SigmaDSP for solid local transient suppression.
Yagami