I tried to edit my previous post to add the following, but the system said I didn't have adequate permission - so please excuse my replying to my own post.
I've partially resolved this. It turned out that the Uplink sound driver was muted both for playback and record. To un-mute these, I used the following procedure:
Double-click the Volume system tray icon The Play Control panel opens
From the Options menu, choose Properties The Properties dialog box opens
From the Mixer list, choose Uplink then select the Playback radio button then click OK The playback properties for the Uplink drive