I went through Jack's points one by one and answered each of them as I went. I 
got almost to the end before finding the answer. Jump straight to the end if 
you just want to see the fix, or read on for the detail.

On Thursday, 24 March 2022 16:40:05 GMT Jack wrote:
> On 2022.03.24 05:55, Peter Humphrey wrote:
> > On Thursday, 24 March 2022 09:43:49 GMT Peter Humphrey wrote:
> > > I'm about to try pavu[...] as you suggested.
> > 
> > No dice. All that does is to allow me to set where I want each sound
> > source to be sent, plus a few details. I still hear nothing, and pavumeter
> > shows nothing being output.
> > 
> > I still can't escape the conclusion that something in my home
> > directory is interfering with sound output.

> Just to confirm - 

Ok, I'm looking at pavucontrol now.

> pavucontrol shows the sound device as properly enabled on the Configuration
> tab,

Yes. The USB device, Unitek Y-247A, is enabled and the other two devices are 
Off.

> shows the correct output port for the device on the Output Devices tab,

Yes: the output port is shown as Speakers.

> and shows the sound source being sent to the right device on the Playback
> tab (with volumes up)

It shows System Sounds* at 100% volume, no matter what I select in the lower 
right corner. The options there are All Streams, Applications and Virtual 
Streams. I don't see an output device on this tab.

> but still no sound from the speakers?

No sound at all.

> Do any of the sound meters show any apparent activity?

Pavumeter shows Front Left and Front Right, with no activity on either. It 
correctly shows Analog Stereo going to the Y-247A.

> Can you see any difference between the pavucontrol
> display as youurself and as the user for whom sound does work?

*  The new user sees Firefox Stream as well as System Sounds on the Playback 
tab. When I logged back in as myself immediately after trying that, I got an 
anonymous error box - and heard its announcing beep! - saying Connection 
Failed: Connection Refused. (Screen shot attached. Does anyone know where 
errors like this are generated?)

> I would guess that if it is something in your home directory, it is
> most likely a configuration setting, probably under .local or .config.
> I see several files under ~/.config/pulse.  You might try moving those
> elsewhere and restarting pulse ("pulseaudio -k" then restart
> pavucontrol)

Lo! and behold! BBC Radio 3 bursts forth!

--->8

Huge relief, and similarly huge thanks to Jack and all others who've helped. 
Now I can go and watch those lectures from Gresham College that have bee 
queueing up.

-- 
Regards,
Peter.