Using the audio library gives the impression that one audio shield can only have two output channels (AudioOutputI2S). However when I studied the SGTL5000 Application Diagram I saw there is a switch that can toggle the headphone out to either monitor line-in or copy the line-out. Taking a look at the audio library source code ( I saw mentioning of the register that control.

Since I am still planning my application I would like to know if there are reasons for not exposing this as a public method. If it for instance behaves unpredictable it might be better to add this circuitry on my own, bypassing the SGTL.