Add this to
/volumio/ap/plugins/audio_interface/alsa_controller/cards.json
at the bottom of the file, just before the Xonar entry:
{"name": "USB Sound Blaster HD", "multidevice": true, "devices":[{"number":0, "prettyname": "Headphone/ Analog out", "defaultmixer": ""},{"number":1, "prettyname": "S/PDIF (optical) out", "defaultmixer": ""}],"type":"integrated"},
Don’t forget the last comma!!!
Note: in case S/PDIF was subdevice 2 (you were not clear, no problem), then replace
"number":1
by
"number":2
in the entry above.
Test this and let me know.