I have just managed to solve a problem where automatic mixed S/PDIF audio output stopped 'permanently' for over a month - the interface was set in non-audio data mode, looks like ALSA didn't flip it back after I watched something that needed passthrough.
You can see the state of this mode with 'iecset', I turned audio mode back on with 'iecset audio true'. This kind of thing strikes me as ideal to be reported on in the Audio Mixer program (so that users don't have to study ALSA to investigate such failures ) - but I have no gstreamer experience etc. Would this be the correct program to change to report on such a thing?
Its now trivial for me to investigate and knock up scripts etc to fix, but it would be nice to make a positive contribution to the GUI to stop this happening for others.