I did a volume control test in the ADAU1450 and found no way to control the volume output (only no output and 3dB 2 results)
You are going around the very long path!
All you need to do is this:
Then connect it to the control input of the volume control cell. This project was more complicated because it also had a GPIO mute button so I just am showing you what you need to do.
You can find the Arithmetic Shift cell here:
This should work much better for you.