The audio adc/dac is stereo only, only has separate gain for line/headphones.
It's likely possible to do lo-fi stereo out with the 12bit DAC inside the microcontroller. Another option is encoding spdif output in firmware (but this will be a clock master, not able to slave to wordclock!). And it is also possible to add another audio converter. But all these options require a substantial development effort, not very high on my priority list.