I have read more of this link and it looks like the solution offered:
xrandr --output HDMI-0 --set audio on
produces the same error message that I have.
So now I have to backtrack.
I know that adding 'radeon.audio=1' to /etc/default/grub switched on the sound on my previous install- and I've added that to this install.
The only thing different between this debian install and my previous one could be that I haven't installed more firmware.
In Synaptic, I'm pretty certain I searched under 'radeon' and then installed all the files that came up.
But these are all fgrlx display drivers.
And I've just removed and purged all the fgrlx files.
Should I try installing them again?