The first one labelled <IEC958>, not C/F/R or U I guess the master for IEC958, it appears as a slider in alsamixer but as a tick box only under gnome-alsamixer. It was gnome-alsamixer that led to me finding it, as it was a tick box, I unticked it (which mutes the channel) and hey presto started working. Some where along the way an alsa specific update must have unmuted it or changed its behaviour.
The first one labelled <IEC958>, not C/F/R or U I guess the master for IEC958, it appears as a slider in alsamixer but as a tick box only under gnome-alsamixer. It was gnome-alsamixer that led to me finding it, as it was a tick box, I unticked it (which mutes the channel) and hey presto started working. Some where along the way an alsa specific update must have unmuted it or changed its behaviour.