Hello rade95

I put a new version of the code in the blog. Somehow copying the html from Arduino to the blog will chop of some code. Also fixed a bug in the volume part. You can just copy the code from the blog, paste it into a new arduino sketch and compile it.
The remote buttons may be reversed. I don't know. I disassembled my OPUS a while back, so I can't test it, just compile the code.

The only way to save the last used volume is to save it every time you adjust the volume (and I didn't want to do that) because Arduino doesn't know when you will turn it off. When you turn it off, it is already too late :-)

The sample rate is actually the oversampling rate of the WM8741 DAC. In order for the DAC to work, you match that rate to the incoming sample rate: high with high, low with low, etc. The DAC cannot automatically adjust to incoming sample rate unless you keep it at high all the time. But if you keep it at high, then you do not take advantage of the internal oversampling and the filters are not in their optimal use (this is according to Wolfson)

The DAC cannot read incoming sample rate. If it did it would not require manual adjustment.