Thanks Capt! Mixer was OK.I found it!
The sound was indeed being sent to the wrong place ...... but not to a different output, but to the wrong section of the HDMI output.
The 'HDMI' card has several outputs ..... analogue, digital and hdmi.
It seems that the default card was the HDMI (only sound card in the PC), but the default device was analogue = card 0, device 0 ....... whereas the HDMI output socket is card 0, device 3.
So I created a text file called ~/.asoundrc
with the following lines
Now the sound is correctly routed to the HDMI socket of the sound card by default.
To find out the correct 'device' of the sound card I usedaplay -l
[lower case L not number 1]
which listed the card and its devices.
Hopefully that might be useful to others who hit this problem.
It has made me wonder why PulseAudio is not (it seems) using the correct device ....