First - thank you Eragon for pointing me to the right road.
I'm late in replying 'cos my electricity supplier turned off the power for several hours. When it cam back I found the same problem with my Duo2. As I'd never let anyone else play with it, I knew that I'd made a mistake when setting both the Sollo2 and Duo2 up.
As the Duo2 links to a CRT TV by SCART, I'd never tried to 'train' the Duo2's remote to operate the TV. And while the Solo2 feeds an LCD TV, that LCD is sold as a monitor with TV rather than a straight TV. When I tried to 'train' the Solo2's remote, it didn't take. As a result I'd never tried to configure the volume settings of either remote. Following Eragon's advice I tried to change the box's volume with the box's remote - no change. And the same result with the remote's mute button. This meant that the box's sound output was being made by the box itself.
I 'trained' both remotes to activate the volume buttons with complete success. Sound works for both box. In case someone can't find the 'training' sequence, I've lifted it from the Solo2's manual here:
Set-up Sequence: <<1 + 6>>, <9>, <9>, <3>, <TV Power or STB Power>
- Press 1and 6 for three seconds until LED is on.
- Press <9>, <9>, <3>.
- If you want to control of TV volume, press TV Power button. If you want to control STB volume, press
STB Power button.